Steam's been my main purchasing portal for years now. I don't think I've ever had real issues with it, not sure if it has crashed even once over the years, although the friendslist has started dropping out sometimes for the last 6 months or so, but usually only for a short while. Never had any issues downloading/installing games, and I uninstall/install a lot.
And as above, you really can't beat the sales sometimes.
Me, I use it mostly for convenience. I like having everything in one place, it's easily accessible, I have a good overview of all my games, install/uninstall is nice and easy like my Sims 3 + expansions. One click + next and it installs all of it, patches and all. Same for NWN2 + exps + patches. Autoupdates, Cloud saves, etc.
There's some negatives obviously.
You won't be able to sell your game since it's linked to your steam account.
You can only log in once so only one person can play the games on that account (you), although that's only true for online games, you can put another computer in offline mode and then you can play the games installed on that one offline just fine.
Autoupdating isn't always good for people who install mods/want to stick to a certain version, or those who have bad internet.
Some people have problems with the offline mode, although I've never had any myself (make sure to have at least once saved your password during login)