Another 24/7 here.
I have always run 24/7 since I believe that it is better for teh hardware (some past IBM hard drives notwitstanding

)
Since I keep 'em running anyway, I do participate in DC project (s).
Systems: 4
Eheim 1048 that has been running almost constantly for a few years.
3 Eheim 1250s of various ages from about 2 years to 1 year.
Computers are turned off for modding and part swaps only.
Reboots are as infrequent as possible- since I use Linux and Win2k, uptimes of months are the rule.