I'm a big fan of Window's hibernate feature. I don't power off my computers I hibernate them. They shutdown quickly and they boot back up quickly - and bring me right back to where I left off. And unlike standby mode (sleep mode to you Mac users), I can leave my computer in this state for days, weeks, months... you get the idea.
Since I upgraded my Laptop to 2G ram (in prep for Vista) however, hibernate has been, well, pretty unreliable. Many times I try to hibernate it either wigs out and drops to standby or gives me an "Insufficient System Resources Exist to Complete this API" error. I though maybe it was something wrong with my Laptop configuration. But I saw the same thing on my desktop when I upgraded it to 2G. Turns out it is a 'known' bug in XP. Well, 'known' isn't the word I would use. I certainly didn't know about it. Anyhow the fix can be found on microsoft's download site here.