Steam's support on OS X is the suck. I don't know what the experience is like on Linux, but given there is less competition from better stores on Linux I wouldn't be surprised to see Steam be a big hit on Linux.
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.
My experience using Steam on linux has been pretty good. The only thing is that the trailers/videos do not work because I run on 64-bit (there is a workaround for this).
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.
What do you find problematic on the Mac version? My office has weekly TF2 games and a bunch of us use Steam on OSX. The only gripe I have is that it doesn't remember my password, but other than that it seems pretty much identical to the Windows counterpart.
Personally, I'm impressed by how horribly implemented Steam's OS X gamepad support is. That takes skill to do, when other apps Just Work™.
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.
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.