I used to leave a few of my computers up 24/7 downloading, running SETI etc.
I even got an uptime of about 3 months with one box.
Until...I saw this:
http://forums.overclockers.co.uk/sho...eadid=17262841 
Not really worth losing my flat, computers and thousands worth of stuff for the sake of getting downloads a bit quicker, now I just leave them on when I'm there.