There were some comparisons a while ago, and it wasn't very close. Working from fallible memory, a C3 might be 30% slower. That's on interger stuff. Floating point is well under 50%. Supposedly they OC pretty good, and run very cool, and may not need a fan, onlya heatsink. They used to say it only worked in VIA chipset mobos, so you better check if you buy one.
That's not necessarily bad. 500Mhz is usually plenty fast enough, for most people, and FP is seldom used in anything other than 3D games. The problem is, you can get a Duron that will run circles around it for maybe $5 more, the last time I looked. Not a great way to save $5, I don't think. Americans don't find absolute minimum systems as attractive as they do in other parts of the world that don't have money to burn. Even the lowest cost PC can be a luxury beyond most in much of the world.