I have about 25 games.
The good: I can easily get them downloaded on any computer I own. The sales are insane. The titles available often include really obscure stuff that's hard to get a hold of any other way. (The void comes to mind, as well as some older games.). Cloud based saves can be useful at times when you're like me.
Steam games don't require me to haul anything at all around with me. (As a laptop owner, that's a big plus; as is simply not needing to swap disks constantly.)
The bad: No internet = no games unless I'm very careful about setup. Again, a big issue for a laptop owner, especially when some games I thought were in offline mode actually had an incomplete 'update' recently, and have thus been temporarily crippled because I'm now in a location without internet access.
Prices when things are not on sale can be scary.
The steam UI/ game manager. It has to run while any steam game does, and sometimes it's a real annoying POS.
Region restrictions - Never experienced it personally, but, as a traveller, the thought that the games I buy ON THE INTERNET, could suddenly all stop working if I move, is a pretty nasty one.
By extension, the fact that it's possible for someone to remotely disable my games...