I've been using Steam since day one (2003?) and I only had a problem once with some obscure old game that I bought on a sale for a few euros. I emailed support and they fixed it the day after (some issue with too many buying it so the keys ran out
). Other than that I think it works great... A few months ago I got annoyed that I had once chosen the default install dir on C: and wanted to move it to my large D: drive. Moved the whole steam dir and was scanning for which settings to fix to get it running - but I tried starting it before doing anything... And it worked like a charm! All games just worked even though I had moved it to another drive "under the sheet"... Nice!
EDIT: My only issue with Steam is the 1 euro = 1 dollar issue that is really annoying...