Challenging and Rewarding
This is easily the most challenging fun I've had for a long long time. The levels are mostly quite short but very rewarding and the gameplay is very addicive especially because of the fast load times and almost non existent respawn times.
This is a game that makes me want to strive for 100%. The last game I 100%'ed was Aquaria, which is an equally awesome game for different reasons. It is a perfect game for completionists.
There are a lot more achievements on the steam version than XBLA. In fact, the extra ones on Steam make it practically impossible to get S rank whereas XBLA is relatively easy. The achievements for steam involve completing every light and dark world chapters without dying. For the first world, this is an easy achievement but for anything past world 3 it's something for the truly hardcore. It's something I don't even want to attempt :p
Most of the levels are so short that it feels ok to die, unlike other games where you die and you have to re-do a whole section again. The warp zones are really the only "unforgiving" levels in SMB, and they still give you 3 lives per level so it's actually not that bad. The good news is that you can choose to skip them and play other levels if you want, unlike some other games that require you to play through a certain punishing section in order to progress.
I think the controls for SMB are the best I've experienced from any game in my life. He does exactly what I want, when I want. Every time I successfully clear a obstacle, I know it's because of my ability to control the character rather than blaming it on "sloppy controls". It's true though that some games with sloppy controls actually create a certain type of gameplay that revolves around the sloppiness, which is a challenge in itself and actually can be fun. But SMB is not one of them :)
The characters offer a nice variety in gameplay, most of the initial characters make certain levels easier to complete, and most of the late-game characters have abilities that can actually compete in the top of the leaderboards. For the majority of levels, Meat Boy is the ideal character but there are some levels where Naija, RunMan, Captain Viridian, Flywrench and The Kid really shine and achieve impossible times. They are all fun in their own way.
I personally did not find Steve (Mr Minecraft) and Meat Ninja to be particularly fun at all. Firstly because they don't have any leaderboard times to beat, which removes any kind of "goal" to work towards and secondly because Steve is a cheater and it really isn't that fun :p Meat Ninja does look pretty fun though, too bad mastering his ability isn't "worth" it.
I still haven't mentioned the awesome level design! Team Meat have done a extremely good job designing these 300+ levels, none of them feel too samey and all of them offer some kind of challenge (except the initial ones of course, they can be excused for being tutorial-levels :p).
There are many retro-references that made me laugh out loud and the overall polished controls, level design, art and music makes an all round perfect package.
The only downside, for the PC version, is that I have experienced quite a lot of crashes during the load screens. I am pretty sure they are being fixed. But in my opinion this is not annoying enough to take away any stars from my rating.
Super Meat Boy is definitely going to be one of my "classics" in the future that I will come back to enjoy again. It's life still hasn't even ended, especially when the level editor comes next month to extend it's lifespan infinitely!