Because of mechanical wear and tear in traditional hard drives and fans, I've never really liked turning off my computing devices. A steady on state is better for the hard drive in the long run.
Just like constant highway drive, stop and go traffic is bad for your hardware. However, all the concern these days for being green and conserving resources have caused me to slightly change my computing habits. Now, I make sure my displays are off before bed, but I still leave my desktop and laptop on almost all the time. The desktop is even protected by two massive UPSes from APC, just in case there's a blackout.
Still thought, I like my equipment instantly accessible at all times.
The question of the day is: Do you turn off your PC at night?