FUCKING LOVE IT. Loved the flash prototype way back when and love SMB now.
The PC launch was a bit shaky but it's been pretty great still so far.
Think of XBLA as vanilla. Whereas PC is Super Duper Meat Boy, with an incoming level editor (January), new, exclusive characters (SPOILER), Tofu Boy (the PETA SMB parody (as of today)) and obviously, faster updates. I believe some characters in the XBLA version aren't in PC, like Tim from Braid and the Castle Crasher, but don't quote me on that.
Direct2Drive version launches tomorrow with today's Steam patch, which fixed a few bugs. Currently SMB PC crashes on exit. Leaderboards can be a bit dodgy at times but it's still very very playable.
Follow @supermeatboy for on-the-minute updates.
Edit: 300 levels over 7 worlds, with a dark version for each world, including "warp zones" which put you into other games/consoles. There will be more levels through the level editor, as well as more chapters by Ed and Tommy, AS WELL AS worlds by indie creators (VVVVVV creator Terry Cavanaugh for instance). There is and will be chapters created around certain characters (there's a Gish one, for instance).
This is the indie platformer to end all indie platformers. It's fucking fantastic and you should buy it now. Try out the prototype to see if you like twitch platforming: http://www.newgrounds.com/portal/view/463241