It has its pros and cons just like any other program.
Pros:
1. Free
2. Auto update
3. Steam Friends (before it was shut down, and then brought up again as a beta, it worked pretty good and you could invite your friends to play chess or checkers or whatever other games it came with).
4. VAC (banning an account is more effective then anything I've seen, IP ban is ineffective, and MAC address ban only works if the guy doesn't have more than 1 PC).
5. Buying games over it means you can never lose a CD since its tied to your account.
6. It helped RO and TW become a success because its cheaper than getting published through retail only. Plus everyone can see other games that steam supports so word spreads quickly.
7. Steam backup.
8. Steam support. Has very useful FAQ, as long as you filter out most of through quick and easy search.
Cons:
1. No refund policy.
2. Downloads are slightly slower than finding a website and dl it from there
3. Buggy (can't go any further, that way madness lies).
4. Uses up extra PC resources.
5. Unless you have offline mode enabled before you internet connection goes down you can't play anything (at least thats how I think it works).
6. You have to install all games into the steam directory, meaning you better pick the best spot when you install steam for the first time or your screwed. Unless you uninstall steam and all your games and then reinstall into the place you want.
7. Steam backup (meant to be both a pro and con since I've had problems with it, could be just me though).
8. Steam support. No live person to talk to.