Well, if you're talking SETI@Home....I'm running that.
When I run it all the time (not just in screensaver mode) it tends to slow my computer down a bit and I usually have to reboot my PC once a night because it just gets sooooo slow. (I havea Pentium 3 at work.)
But it doesn't do that on my parent's Pentium 4 at home where I can run it all the time without a problem.