Some gamers still prefer PS/2 if given the choice, even though there's little technical advantage. Although their most cited reason (key rollover) isn't valid; the keyboard determines how many simultaneous key presses it will transmit over USB.
PS/2 keyboards observationally are known to have a bit less latency. Since there are a ton of variables at play (keyboard controller, USB poll rate, PS/2 interrupt queuing, etc.) it's hard to evaluate just how much the difference will be with any given setup.
My understanding of how n-key rollover was finally implemented in USB keyboards was by putting a USB hub in the keyboard and having the keyboard pretend to be several keyboards all at once.
