Valve is good but overrated IMO. I still prefer GOG to Steam honestly enough.
yeah, two things GoG has over Steam are game compatibility and drm free installation.
Steam is best of breed for its kind of service, but the program itself is the drm. What gets me to go with Steam over GoG for newer games are the features (cloud saving, achievements, automatic updater, etc.) For older games like Thief 1&2 I'd rather go with GoG.
I don't think Steam is overrated, though. There's that story recently about vastly improved sales for some indie game once it hit Steam. A lot of people treat it as the only PC platform because of how convenient it is. And Valve manages it really well.