I have several Steam games installed, and I've been running Steam and Vista together since Vista launched. I've had no problems at all, and DirectX-based games (Source engine) actually seemed to improve in performance when I switched from XP to Vista.
Steam isn't Aero-enabled, but it doesn't force the system to switch to Vista Basic's color scheme, either.
The DRM system works well, and it's not too invasive. You can sign into your account from any computer that has Steam installed, and it will download the necessary files for you to play the games on your account. You can make CD/DVD backups of the titles
you've purchased. And you only need to run Steam when you want to run a game you've purchased with it, so it doesn't need to be running all the time, like at system startup. There are no background processes associated with the program when it's not running,
as with iTunes.
The latest build of Steam introduced some really nice community features and a great in-game UI, as well. And the game selection is incredible. I say go for it, you won't be disappointed.