I've seldom misheard a controller due to their accent (although I have misheard placenames, ref
another thread, but I don't really think this was because of the controller's accent...).
Surely more of an issue for you guys is the accent of foreign pilots? I have sometimes heard some Europeans (not the majority, just some) and others being handled on radar frequencies when I have also been on the same freq and have been impressed at how the controller has been able to decode what the pilot was saying!
Andy