My review for DiRT2 is up. Very good game, though I think the genre is inherently always catering to a core audience. Casual racing games with undemanding physics opt for eccentric courses and probably a fair number of wacky power-ups to keep it interesting. This game is both a nod to catering to a broader audience while still keeping the core principles in place: good driving is required to get anywhere in this game.
Lots of variety in the race courses and what is easily the most beautiful scenery I've seen in an Xbox 360 game (though I understand it's easier to pull this off in a racing game) are all nice additions as well.
If this is your thing, you need to get in on this game.