TL;DR: 8/10, would use as an alternative for an adrenaline epipen.
So I bought this game not really knowing what to expect. I'm into racing games, but mostly the Need For Speed series. This game is certainly the most unique racing game I've played. Keep in mind that I haven't played NFS: ProStreet or Shift, so I can't make a comparison with those games.
GRID is a racing game that attempts to simulate a professional's driving career, and in my opinion, succeeds for the most part. You're allowed to create your own team name, colors, and run just about everything else. You can buy cars, hire drivers to gain more points for your team, etc. However, you never seem to pay for damages taken to your car, which might irk some who want extreme realism. You also have the option of accepting other teams' offers and driving for them to earn some extra cash. Along the way, you'll gain sponsors, which you can put on your car to gain extra money if you meet the company's requirements for the race (e.g. Get above 2nd, win $4K).
The biggest aspect of the game is, of course, the driving. I'd say it's one of the best handling and realistic driving mechanisms I've played, especially for PC and using a keyboard. The options menu gives you control over how sensitive key presses send your car so that you're not constantly hitting left and right to straighten your car (although you still have to do that sometimes, but not often). The stats of each car accurately correspond to how they handle on the track, and there's a variety of classes and brands to choose from.
One thing I'll say, if you're used to racing Need For Speed, get ready for a rude awakening from the world of realist driving games. You can't just press up and bounce off cars and walls constantly to get a win. If you hit things, your car will take damage, and the way it's shown on the car is very accurate. Take too many hits and you'll start damaging things like the wheels, transmission, etc. If you slam into something too hard, you're out of the race. Braking is very imporant in this game and it'll take a few tries (and wrecks) to get the hang of it, but after you learn, the game is a blast.
So far, multiplayer hasn't worked for me. Perhaps they took it down. Even if they did, though, the single player is enough to keep you extremely entertained and have your heart pounding. The AI is very well programmed, trying to avoid hitting you and taking routes that a normal pro driver would. The tracks are very nice, with some (I'm not sure if all) using real-life tracks. The soundtrack is very good too, although you won't notice it most of the time, as the driving takes most of your focus.
Overall, a very good racing game. There are a few nitpicks I have like how your spotter sometimes says, "You took some damage, but it's nothing serious.", even though your car can be severely damaged. The graphics can also wear on your computer, so if you have a sub-par graphics setup, then make sure you can run the game first before buying it. They look very good though, even on lower FPS the game looks to run smoothly. Like I said, an adrenaline rush that is worth the money (even moreso if it's on sale).
(Appending this to the review, apparently if you have to reinstall the game on another computer, your campaign data is lost and you'll have to start over. Removing .5 points since I don't know if it's the game's or the producer's fault.)