I've spent a considerable amount of time with Grand Theft Auto IV, and I think it is probably the best game I've played in a very long time.  We aren't even bothering to write a single player review because there are dozens out there, and the Metacritic score speaks for itself.  I can assure you that aside from a few nuances, GTA IV is worth every penny on the single player side of things.  Go buy it.

However, after everybody finishes the single player campaign and has had their fill of running around Liberty City by themselves, it will be the multiplayer aspect of GTA IV that will determine this game's true longevity.   You don't find too many reviews on the net that have concentrated fully on this experience.  As such, this review is more about the multiplayer side of things, and how GTA IV gets many things write, yet gets some basic things wrong.

If Rockstar patches these issues, GTA IV will displace Halo 3 and Call of Duty 4 as the new king of multiplayer.  But if they do not fix these issues, GTA IV will prove to be too frustrating of an experience for many players.

Grand Theft Auto IV Multiplayer Score:  ★★★★☆

What's right:

The game modes are fun.  The very nature of game ensures a good time, and Rockstar definitely spent a considerable amount of time making a few game modes an interesting and fresh experience.  The standout mode is definitely Cops and Crooks (although everybody online calls it Cops and Robbers...), and with up to 16 people shooting it out and racing across Liberty City, all hell breaks loose in a way that every gamer needs to experience.  It is simply amazing.

Pedestrians being run over, light posts falling down, tires being shot out as you desperately try to make it to the escape helicopter with the cops one block away with sirens blaring...  It sounds frantic because it is frantic, and best of all, it is exhilaratingly fun.  There are other modes as well, such as team deathmatch and a form of territories, and they are all great.  Even just the mode where you run around the city with no goals is fun... because the core gameplay and Liberty City itself is fun.  It's a formula that you just can't get wrong, so what is holding back GTA IV's multiplayer???

What's wrong:

There are a few issues that continually plague multiplayer matches.  It always baffles me how 75% of multiplayer flaws are found in the first 10 minutes of gameplay, yet developers either ignore these flaws, or have really retarded QA departments.  It makes you really want to start a multiplayer QA company.  But I digress...  In GTA IV, some flaws are more critical than others, and here is a numbered outline of things we feel are the most painful:

1)  For starters, in keeping with the do-everything-from-your-cell-phone theme, Rockstar forces you to use the cell menu to start your multiplayer adventures from inside the single player campaign.  This isn't itself all that bad of an idea if it weren't for one critically obvious flaw.  You have to load the single player game up in order to access the multiplayer part of the game, which in turn has to re-load the game to start.

This is retarded.  Why be forced to sit through two loading screens just to play multiplayer?  I really feel Rockstar should have put up a cheap menu so you can select between multiplayer and single player before anything loads.  If they wanted to keep the flow of the game as-is, they could have at least added this option after you beat the single player campaign.

2)  You can't host a private match.  In fact, you can't even choose to host.  [Edit] Thanks to user tips, we've been enlightened on how to host a private match.  Pull up your cell, go to custom game, and set the private slots to > 0.  That will ensure that you host the match, and if you set the number of private slots to the max, it will be a truly private match.  Though the functionality technically exists, it would have been much nicer for Rockstar to make it more apparent and user friendly. Thanks to zx6 for pointing this out.  It should also be noted that if you want to change the number of private slots, you have to tear down and re-create the room again, which requires loading the single-player mode again, just to access the cell phone to access the multiplayer (see complaint #1).  Lame.

3)  Unless you are hosting, you can't invite your friends to play with you, once you have joined a match.  No really... you can't.  Say you find yourself on a really fun server with a great group of random people, and you want your friend to join in.  Well, they can't.  Sorry. There is simply no way to invite them in.  You pull up your friends list, and the "invite to game" is strangely grayed out.  Damn.  But fear not, there is a way to play with your buddies, which leads us to our next problem:

4)  The party-system is half-assed.  Kudos to Rockstar for putting in a multiplayer party system.  I cannot thank them enough for this.  If you want to play as a team with your friends, you totally can, and it will not split up your team.  Wonderful.  The problem is, the party system is very one-dimensional.  It feels like it was tacked on at the last minute, and never made it out of beta.  Let me walk through how to play with your friends in GTA IV:

Step 1:  Wait for the single-player mode to load, then use your cell phone to pull up Multiplayer, and then select Party Mode.  Wait for it to load.
Step 2:  Use your cell phone to invite all of your friends to the game, wait for them to fully load into the world (a sort of sandbox mode).
Step 3:  Figure out what mode everybody wants to play, select the appropriate mode, and finally:
Step 4:  Load the selected mode, and pray that everybody makes it onto whatever random server is chosen for your party to play on.

It is step 4 that is the shittiest part of all.  Sometimes it works, but many times it doesn't.  Right now I am witnessing about a 30% success rate in having the first attempt actually get my entire party onto a server that we can actually play on.  When it doesn't work, it appears there is a variety of reasons.  Sometimes you'll be put on a server where there wasn't enough room for your entire party, dropping the rest of the players back into single player mode (which they have to sit there and watch load) along with an error message saying they were disconnected.  Even if the server has open slots later on, you can't invite your friends that were left behind (see problem #3 above).  You have to go back into Party Mode, and invite them from there, only to attempt this entire process again.  Now, assuming you all finally make it onto a server together, it won't be long before you discover the next flaw:

5)  There are flaws in how the teams are balanced.  Team balancing is done once, right as the lobby is being loaded.  If half the people on the other team quit after the lobby loads, chances are you are going to be stuck with uneven teams until you play a round (and the smaller team gets slaughtered) and then re-load the lobby so balancing can occur again.  Sometimes if you wait long enough, random people will join to balance out the teams, but this can take an unnaturally long time.  Of course, this could have been fixed by adding a menu that would allow you to change teams, but if you are in a party, it isn't there.  You read that right, there is no way to change which team you are on if you are in a party.  It's comical to hear the host screaming in futility for people to "drop down" onto the smaller team.  Little do they know there is no way for party players to do this, and even better, nobody can tell who is in a party together!  Want to split up your party and go against each other?  YOU CAN'T.  Hopefully you like playing with all of your friends, because chances are you won't be playing against them unless you pick a free-for-all mode or host the match yourself.

6)  This bug is pretty huge. The co-op mode called Bomb the Base II is basically unplayable.  If you have more than one person playing (say, you and a friend), each of you get a limited number of lives.  If one of you dies too many times, the mission fails (if you are playing with 3 people, 2 people need to run out of lives for the mission to fail).  This in itself is fine, however if the other player still has lives left while the mission fails, they will be stuck staring at a screen waiting for the lobby to load, eternally.  The player that lost all their lives is properly loaded back into the lobby, however the player with some lives remaining is stuck staring at the boat.  You can't move, and you can't choose any menu options other than quit.  Nice one, Rockstar.  So basically with this mode you have two choices:  win, or re-load the single player so that you can re-load the multiplayer to give it another go.  WTF????  So much for QAing this mode, I guess...

In addition to these problems (and the complete lack of user-friendly menus that make sense), there are other usability issues with the party system that Rockstar has created for this game, which we could probably make into another list all by itself... for example:

Want to select a different server for you and your team to play on?  Well, you have to wait for the right phase of the lobby system to load before you can do that.  This occurs after a game has been played, and after the host has selected the next match type, which then triggers the next lobby to be loaded, at which point at this exact time you can hit the "Back to Party Mode" button, which triggers more loading as you pull up the sandbox world again, and wait for all your friends to re-join your party world.

It all sounds very confusing because it is, and yes, there is a lot of shitty loading, all for nothing.  It's like Rockstar took all of the loading problems that used to plague the GTA series and stuck it all in multiplayer.  The single player part of the game has almost no load time when compared to multiplayer for GTA IV.  The whole system needs to be re-vamped and why no other developer can mimic the seamless beauty of Halo 3's multiplayer, I'll never know.  Call of Duty 4 comes the closest, but they still didn't get it right, and neither did GTA IV.

As it stands right now, it's just a little too hard to play with your friends online in GTA IV.  But it is very fixable.  Rockstar, get on it, and you'll have the greatest multiplayer game of all time as part of your back catalog some day.

Don't get me wrong, I'll still play the multiplayer in GTA IV to death... it will just take a frustratingly much longer time to do so.

Additional thoughts:

When we were reviewing this game, UserX 101 brought up an interesting point.  He felt as though the multiplayer for the Xbox 360 version probably suffered because Rockstar had to create a menu system that would work for both the 360, and the PS3.  He felt that developing multiplayer for the PS3 (and hence, making the 360's version match) probably held back the Xbox 360 version.  I don't know if this is true, but when navigating the menus and using the lobby system, you can't help but get this feeling.  It feels very much like a PS3 multiplayer game, and nothing like a proper Xbox 360 multiplayer game.  UserX 101 might be on to something...

It would be interesting to hear from Rockstar on whether the PS3 had any influence on how they designed the multiplayer lobby system.  In the meantime, we all suffer.