I've tried it. It doesn't work well. It's easier to play those games with controllers.

The problem with those two games is that you're supposed to be pressing only one or two directional buttons at any given time in both of those games.

Directional inputs are single direction at any given time. It doesn't make sense to be using multiple fingers, because when you're switching direction from moving left to right with wasd controls(for example), you need to release the a key after starting to press the d key. There are two movements you need to do:

1. (while pressing a)Press d

2. Release a.

That's two separate finger motions, compared to just shifting the stick to from the left to the right, or shifting your thumb from left to right.

This is especially compounded with fighters like Street Fighter, where directional stick movements are important. For example, to do Ken's Hadoken, the motion is a quarter circle forward(down, down-right, right) then punch. With a d-pad or fighting stick, this is fairly easy to do. Thumb/hand moves down, then shifts right and up. The other hand then presses the punch button.

For a keyboard, it's:

1. Press s.

2. While holding down s, press d.

3. Release s.

4. While holding d, press the punch key.

So 4 discrete finger motions with 3 different fingers. Again, compared to a fighting stick, which is two motions with your hand(down, and basically move hand to the right, following the side guide), and punch. That's 1 less motion, and 1 less finger to care about. While it's certainly possible to train your muscle memory to do it, it's much easier to just use the proper controller.

There seems to be a consenous(ish) among players of Super Meat Boy (a mario-like, very hard, platformer) that using a controller is slightly easier.

Certainly I have tried playing street fighter with a keyboard and found it extremely hard.

Also, I couldn't imagine how you would play a 2d platformer which used an analogue stick. You lose the analogue if you map to the keyboard, and it doesn't really map easily to a mouse (as you can't keep moving your mouse in one direction forever).

You should try using a stick with Street Fighter. I can't pull SF moves off very well with a pad, but with the stick (especially if it has an octagonal gate), it's super easy.

There is something called a "Hitbox", which is a fighting controller that is all buttons: http://www.hitboxarcade.com/

I think a few people at EVO were using hitboxes.