Is it okay to leave my computer on all the time?

Short answer

Yes, but in moderation. Restart your machine once every three or four days to avoid problems.

Slightly longer but more informative answer

Yes, we know it's a pain to have to spend a few minutes watching the pretty loading bars when your computer turns on or off. Unfortunately, both Mac OS X and (especially) Windows tend to develop nasty behavior if they're left running for hours on end without an occasional reboot (that's right Mac users, you're not immune, just better off than Windows users...as much as that means...).

The problem is usually memory--system memory that is. While both are good operating systems, neither Windows or OS X are terribly good at managing your memory over extended periods of time (unlike some other operating systems such as FreeBSD). If your computer is left on for long periods of time, the organization of your system memory tends to get mangled, resulting in weird behavior and random errors. However, since your memory is wiped clean every time you turn off your computer, a quick restart should solve these problems.

Also keep in mind that you are also subjecting your computer's hardware to extended period of use without any "rest." Granted, most of this time will be spent idling (which most computers now take advantage of by going into some "low power use" mode of one sort or another), but the operational lives of your computer's fans, power supply, and capacitors will still be used up at a faster rate.