I bought this game after downloading the XBLA demo, and I have to say I'm quite satisfied. I enjoy a solid platformer from time to time, but the indie hype train was quite the hurdle to get past before I would even get this game a shot. I'm glad I did.
I spent roughly two hours and under 500 lives (holy shit) just going through the first level (light & dark) and most of the second level. I can't tell you the last game I played where my thumbs were left so sore after such a length of time. Obtaining all of the achievements will be a pleasure, granted I'm not thwarted by the time the later levels arrive. It's amazing to thing how difficult the Steam achievements will be! As most people in this thread have noted, the game is insanely difficult but appropriately rewarding. Since the levels don't last long, dying isn't as frustrating as it would be in other games - it makes the game relatively more accessible, while still retaining it's tough-as-nail mechanics for speed runners and such.
The controls are a big part of the learning curve in this game - at the end of two hours of play, I'm finally getting a solid feel for how the analog stick handles. I would try out my properly d-pad-equipped Rock Band controller, but the right trigger seems really necessary, rather than fumbling my way through three face buttons all of the time. It might be worth it to check it out if anyone else here has had success with this setup on Super Meat Boy.
I'm hoping the Wii release is disc-based, because I won't think twice about double-dipping if it is.