I play on an Xbox pad but it has it's problems. The d pad being on a divot makes rolling inputs pretty comfortable on the thumbs (and for me more reliable) but sometimes it messes up your inputs in the cardinal directions.
Forward or back are fine on their own. Back, forward however is another matter. If you have inputs turned on you'll see the pad will cough up something like 4136 as opposed to 46. This isn't really a problem though (most games will understand) until you have a fighter with inputs for moves that comprise both motions. Then it becomes a royal and unforgivable pain in the ass. Having used an xbox pad for over a decade I still cannot reliably play Makoto in BB or Ryo in KOF using the standard xbox pad. Why? Because a good chunk of the time I get the wrong move. Ryo's Zanretsuken and Makoto's Space Counter are not something you want to try with the xbox pad in a pinch. You'll do your best to input 646 and wind up with a back to forward roll or a "ride the lightning" 632146 input. And since both of them have a meter eating move on ride the lightning that's a bad thing.
At least that's my experience. I'd say 90% of the time it's not an issue. But if those are your characters then it's an unavoidable nuisance. And since I find it hard to make excuses for that sort of thing I've been looking into alternative pads.