Yes, DiRT is totally worth $2.50. In some regards it's actually better than DiRT 2, simply because of the variety of tracks, heavier emphasis on rally, and less of the cheesy "X-GAMES" feel to it. (I haven't played 3 yet.) Get it if you don't have it.
In case you click through the purchasing pages too quickly, there is a warning that this game is not compatible with Windows 7. A google search shows that there are some ways to get it working with Windows 7. For $2.50, I'm taking a chance that I can get it to work.
I'm able to run it on Win 7 quite flawlessly, no tweaks required. I think Steam says "Not compatible with Windows 7" on some older games just to cover their asses in case it really doesn't. DiRT works fine though.
I've heard some things about it not utilizing multiple cores, or something like that, but it's an easy fix.