I've not used a cycle computer, but I have used a GPS! I have a Nokia N810, which paired with Maemo Mapper can show you your speed, altitude (less useful on a C5, granted) and record your route, along with distance travelled. It can also dump everything to your computer. Obviously this is a little bit of an overkill, but it's by far the simplest way to go, as you don't need to do any sort of calibration and setup that you do with a cycle computer. Although I can't see why one wouldn't work!