Win 7 64-bit:
First, the game works fine.
I got some Achievements from the starting screen & Credits scene too.
Tobas Chilli, Knytt Stories and Because It's Tradition, all of them works fine.
But there's a minor complain about loading at the very beginning of the game.
Steam version takes longer (about 5s I think) than standalone version.
(I have no idea about this, it could be a specific problem. The splash screen of Steam ("Press Shift+Tab to open...") might be the cause.)
And loading between different rooms is also slightly slower than standalone, but it doesn't feel that much, maybe it's just my feeling, anyway, it's OK
++++++++++
Ubuntu 12.04:
Linux build is even better than any Windows build! The game runs fluently and loading is very quick!
Here's one little thing: you could actually change the window size in Linux build. I don't remember I could do this under Windows
Game itself runs great though, but there are some problems with Steam overlay under Ubuntu.
I pushed Shift+Tab to call out the Steam Community page, then I found I can't click those buttons or links (actually even the mouse cursor is still an arrow not a hand if a link is there normally) , but sometimes I can open them, after clicking them several times in game window.
I also got EDGE and FEZ under Ubuntu, their Steam overlay function great.
And the cloud sync works great.
P.S.: Just in case, here's my spec: My laptop is ACER 4750G, NVIDIA GT540M & INTEL HD 3000 (built-in CPU), 6GB RAM (using Bumblebee upon Ubuntu 12.04 LTS 64-bit).