I still don't quite trust it, but that is mostly about the possibility Valve could do something nasty to my games in the future (or go under and take the Steam servers with it). And you better have a broadband connection, as Steam tends to do a lot of patching. So TEKing66 has a point there

But back to how things developed:
When Half Life 2 was new, the inadequacies of Steam caused quite a furor on the internet. Lots of people cursing Valve, and that was probably deserved.
IIRC one or two years later I got a copy of Day Of Defeat: Source. At the time, things were better than at the release of HL2, but outages were still frequent. Especially the Friends network was often unavailable.
Today, it has become rare that a Steam game won't start. Very occasionally I get a "this game is currently not available", but it is usually fixed after 10 minutes. So I have to admit that Steam usually works fine.