It plays okay. The d-pad on the screen can be a little annoying, particularly when jumping off of ropes; but the controls are not "bad." On occasion, I've tapped the jump button when I meant to attack. But I've never died because of the controls, although I've fallen off a few ledges.
The game is great, but the controls are just serviceable. I don't have a DSi, so I'm just happy to play this game on another platform.
If you have a DSi, then I would get it on that platform because the control scheme is superior. But I paid $2.99 and I'm totally happy with the game.
Last games I beat: Deus Ex: HR, Catherine, Bastion, inFAMOUS 2, LA Noire, Might & Magic: Clash of Heroes, Portal 2
I'm playing on the iPad and I think it plays well. I've played enough iOS games to be used to virtual controls (multiple plays through Illusia taught me platforming without buttons). Every iOS game gives the controls slightly different sizes and puts them in different places, so there are times where I find myself tapping next to the button. Having said all that, the game seems to be forgiving enough to make do.
I just beat it. This is the best platformer I have played on the iPad. I cannot say I had trouble with the controls, but I am used to virtual controls. I you are used to tactile buttons you mayy have trouble with the virtual controls. The iPhone / iPod touch version may be harder to play (or maybe it is easier if you have small hands?).