I'll go out on a limb and say I liked it. The physics have been much improved over the original, the graphics are excellent, and it really does the "race day experience" well. Cars actually have a sense of weight to them now, and I found the A.I. challenging even at the mid-level difficulty setting.
What you think of the game will depend on what type of rallying you're into. If you are expecting a WRC-like experience this isn't for you. I really missed the classic '70s, '80s, and '90s era rally cars from DiRT - sadly they are nowhere to be found in DiRT 2. Speaking of which, I think DiRT 2 has less cars than its predecessor, which is a bit embarrassing. I don't know what Codemasters' DLC plans are but there's no excuse for taking a step backward like that in this generation.
However, if you watched the X-Games and liked what you saw, this game is just for you. DiRT 2 is based around that whole "American extreme sports rallying" that's come up over the last few years.
So the appeal of the first game is kind of gone in the sequel. But the physics are so far ahead of the last game that, at least in my opinion, it makes up for it. That said, I hope Codemasters does come to its senses and give us all the type of game that we want - no X-Games, no characters, just good old fashioned rallying.