Be aware also that USB carries with it a significan't overhead for sending each byte that does not exist for older interfaces like a serial or parallel port. I've only measured USB performance against serial in one case, but serial MIDI performance was significantly more accurate and faster than USB. Given the choice I'd advise people to use serial or parallel ports over USB for MIDI, at least for the near future. If you don't have a choice then get a reall fast CPU that can blast through all that extra code faster.
For Audio... I shudder to even think...
USB has the potential to have very fast, very accurate audio. It's designed for it. But I have little faith in companies actually using those features properly that are needed to make audio fast and accurate. As usual USB is really cool technology, but is so feature laden and multi purpose that it is uncommon to find an implementation that is as fast as a dedcated PCI device, serial port, or parallel port. Also as usual, ignore the marketing hype newer is not always better (at last not at first).