Yes... but you need to qualify that. The mechanic at the bike shop I go to races a fully rigid singlespeed Jamis MTB in XC. He blasts everyone on the climbs but gets killed when the terrain gets gnarly because he has to pick his lines better while the guys with the FS rigs and any amount of travel can just blast right on through. Also the fact that he's only got one gear means he runs out of them on fast descents. His strategy is to try and put as much time on them as he can during the climbs and when the course is relatively flat. The problem is that if he laps them (and he does quite often), he'll be stuck trying to pick his way through the pack without suspension to shoot straighter lines.