For me, space was a big issue, I still have yet to find a timer that doesn't take up more than one outlet worth of space on my powerbar. I ended up getting one of those garrison timers from crappy tire but the thing is huge. Its nice having the digital readout and battery backup though, I don't have to fiddle with it every time the power goes out.
Edit: Oh ya and as Der Iron Chef mentioned, make sure you have the three pronged one! I have a small pile of two pronged timers due to my lack of attention when purchasing
