I sleep mine, I don't think its been "off" off since I got it. Last time I checked, my powerbook G4 hadn't been rebooted in over 34 days. I only reboot that when an Apple update comes out. I can't remember the last time it was completely off.
In contrast, I just got a new IBM Thinkpad T60p laptop at work. I've been leaving it on and I had to reboot it after just 4 days because random stuff started to crash
Don't get me wrong, I'm not a PC hater, but man, that's annoying.