I know I really should turn it off, but I leave it running if I'm returning in less than 4 days... If I know i'm gone more than that, i'll turn it off. This computer looks like it will die soon, because i'm never off it.

I leave mine on unless I need a reboot for some reason. The only reason I do this is because I deal with a lot of people in different time zones and sometimes they need to get in touch with me at odd hours. The computer is in my bedroom and in an important situation they know they can nudge me on msm to get my attention.
The only problem I see with leaving it on is the added electric it uses. I don't think heat buildup is a problem if you have sufficient airflow. Under idle, the computer should reach it's operating temp and stay around there, if it creeps up, you have a cooling problem.

All the time. Mostly at night or when I don't use it.

I usually leave my computer on since it is a hassle for me to wait until it actual shows up to the desktop and gets down loading plus easy access to it while it is on and still connected to the Internet.

omg, my 5 year old comp just overheated

That does not sound like a good thing, Dan. I hope it is able to be useful for a bit longer. I would hate for one to buy a new computer after having it for such a good period of time.

I turn my computer off after each use.