To put it succinctly, if you like racing games, GRID is a must-buy.  If you don't like racing games, GRID is still most likely the game for you.  It is the kind of racing game that even people who hate racing games will love to play.

If you were to have the PGR and Burnout family lines cross to form an offspring (with maybe a little Forza adultery on the side), GRID would be the outcome.  It's fast, it's exhilarating, and there is just enough realism there (with the crashes and drifting physics) to pretty much make everybody happy, even if you are a Forza purist (the crashes and graphics will woo you, and mastering drifting is challenging).

So far it's a great game, and there are many online modes (ranging from demolition derbies to freestyle drifting courses) to keep everybody happy.  The single player mode is very thoughtfully laid out and it keeps things interesting with different challenges almost every race.

The tracks and environment graphics in GRID are arguably better than any racer out there, with the car models falling just south of PGR's, but not in a detracting way.  They are actually quite good, and I prefer the cockpit view in GRID over PGR's.  The framerate is rock solid, and the sense of speed rivals or even bests that of Burnout Paradise (in fact, it feels like you are playing a racing game in fast-forward sometimes).  Although artistic lighting of the graphics engine suffers from the permanent orange sunset we despise, it doesn't hurt the game too much.  Just crank up the brightness a little.

Download the demo, it is a good taste of what the actual game is like, and if you like the demo at all, you'll love the game.  Buy it.