From a single player perspective, DiRT 2 is overall a good game, but severe lack of variety lets it down
The one area that let's the rest of the game down is lack of track and event variety. This may not be a huge issue for online multiplayer, but for the single player you'll get bored long before you complete the game's lengthy career mode.
The problem is that there are only about seven or eight tracks, with a couple of variations on each one, and the game forces you to race each one to death. By the time you're half way through the career mode, you will know the majority of the tracks so well that there is barely any challenge left. Add to that the fact that the environments you race through are all very samey - dusty deserts and scrubland that is all the same colour - the two notable exceptions being London where you race around a power station, and Malaysia which is a jungle track.
This really felt like a lot of wasted potential to me, because everything else about the game is brilliant (except for some of the voice sound bytes which can be corny, banal and downright irritating - but thankfully can be turned off). Hopefully this severe lack of variety will be remedied in DiRT 3.
Anyway, for the single player, DiRT 2 is fun for a few hours, and definitely worth playing if you're into off road racing. It just fails to keep you interested for the 15 + hours it takes to complete every event.
It's worth mentioning that it is entirely possible to only do the rally events, which are the most fun, and skip the truck/buggy races altogether, and still make it to the end of career mode. I would personally recommend this tactic.