I've heard about the potty problems as well, although I believe it was more along the lines of the cats that had really short tails, and if there had been spinal injury, which could lead to bowel control problems or habits.

I had a manx when I was younger, and she was great. She bunny hopped a lot, and had a bit more of a stubb of a tail. They tend to be more boxy and compact in build than other breeds. All in all, very nice cats.