I've been playing this. Here are my impressions, given I'm about 60% in (World 4 of 7, looking at the map).
Starting with a positive, the controls and physics are wonderfully fluid. It's straight out of Super Mario Bros. (run button, jump button, inertia), plus wall-sliding and wall-jumping. Like Mario, there is a smooth analogue-like feeling even though the controls are digital. There are unlockable characters with varying abilities that make things easier, but (I think) all move slower than Meat Boy.
Enemies are rudimentary path-walkers and homing missiles, predictable and consistent. Since you can't fight (at least not before World 4), the only way to deal with enemies is to avoid them.
Elements of levels are also rudimentary. Every surface is at a 90 or 45 degree angle, static or moving in clockwork fashion. The only complex physical object is the player. There's not much you can do to change the environment. The only way of interacting with anything is bumping into it: collecting items (e.g. keys to remove lock tiles), crumbling walls (tiles that dissolve a second after you touch them, less satisfying than smashing bricks with Mario's head), leaving blood everywhere you touch (actually serves as a useful marker), and getting instantly killed by most moving objects.
Instant death is very welcome. Individually, the levels pose decent, short challenges. Successfully executing a string of wall-jumps and skimming past obstacles is very satisfying. With Meat Boy, that is. Using Gish (sticks to walls and ceilings) or Commander Video (floats) feels like playing on easy mode. Getting some bandages (these unlock characters) requires the abilities of certain characters.
Unfortunately, the game structure nullifies much of the overall challenge. Within any unlocked world, you can tackle any of the 20 standard levels at will, as many times as you like. The levels are so short that this has the same effect as frequent checkpoints, as Cephissus alluded here. So when it gets tough, you'll grind away and win (or get a good time, or pick up that bandage) sometimes by your own improved skill, and sometimes by sheer brute-force-enabled luck.
To reduce the impact of fluke success, limited lives would be a definite improvement. Or limited time per world. Then you'd be rewarded and scored on your performance in the world as a whole, making a requirement for a consistent chain of successes. Something like the structure of Super Monkey Ball (minus infinite continues). Meat Boy's main mode is like Monkey Ball's practice mode.
There are hidden sections, the retro-styled 'warp zones', that do impose a lives limit (and remove choice of character). Longer end-of-world levels serve as a sort of boss (still no fighting, just a unique enemy to avoid and/or a time limit). Completing a level with a good time (by means of skill or luck) unlocks a 'dark' alternative version. This might mean playing it backwards or adding a dozen circular saws.
Despite the flaws, I'm not bored or frustrated with the game yet. I'm aware that what it does right has been done before. The dark humorous style definitely fits the game. I especially thought using piles of used syringes as spikes was a nice touch. Music's all right too.
