As for me, after being a steam user for years, but given the fact that I haven't been able to play team fortress for 5 months now despite playing it for years, in the future, I'll use the Mac App Store.
As for the games, a lot of hit and miss. Team Fortress works great! However, a lot of other games require workarounds even though they have official linux support. Luckily, the Steam Linux community is quite strong and these workarounds were pretty easy to find.
I really wish Ubuntu and other linux distros can make an app store (or software center) of similar quality. It's really a joy to use.
Also, Steam.app used to suck heaps of CPU just idling in the background, often enough that to make the fan audible in laptops. That improved lately, but still isn't perfect.
Not to mention the frequent crashes when you quit. I guess it's quitting either way, but still.