I wish they would have kept the Kenyan accent on Storm. So what if Halle was bad at it? Get her better training, and if she still can't do it right, at least it's better than turning Storm into nothing more than a white-haired Halle.
I'm glad that Rogue's southern accent wasn't overwhelming, but I would have liked her to have kept a little of it, and not lapse in and out like she did.
I'm glad that Magneto had the voice he did. Ian McKellan was perfect in every way.
I'm glad Pyro didn't have an Australian accent. That way, he's more of a regular "troubled teen" that Americans (the prime target audience) can identify with.
Colossus, I really don't care. No accent is fine with me. After all, shouldn't the majority of the students at the Xavier Institute be Americans, since it's an American school?
Gambit, I would like to see with an Accent. His Cajun roots are just too important.