While poking around on Youtube I found many commenters saying "yeah, now try that with a keyboard." The game itself, however, only seems to suggest a controller to keep the "feeling" of the game.
I played with the keyboard so far, and I'm 10 levels from completing world 5. Sure, my completion rates for worlds 2-5 are below 30% and I only have 11/100 bandages so far – I still do value my sanity – yet I'd rather think it's me not being good enough, than thinking I'm bringing artificial difficulty onto myself for not wanting to buy a controller.
The only gripe I have with the controls is that you need to release Space after having gained enough elevation before you can jump again by pressing Space again. I'm not sure using a smaller button, or a different finger, would make this any less frustrating however.
Is Super Meat Boy actually harder to play with a keyboard?