I've got a cheapy (about £5) one from Tesco. Had it just over a year, worked OK so far. It does current speed, and then figures for your "trip" (you can reset whenever you want) of distance, time, average speed and then a "forever" setting of total distance and maximum bestest ever speed.
I plan to upgrade at Christmas (good ole Santa) to a Cateye v2c 'cos I want cadence and the ability to time laps (well, actually the time between certain points on my trip to / from work) and review the data after. That's about £70.
Or I might go completely mental and get a Garmin Edge 305 which does everything the Cateye does, plus has USB connection to your PC, GPS (not maps, but you can load routes onto it), the ability to race against a previously set benchmark and all sorts of fun. At about £150 !!
Then you've got anything and everything in between. As said before,
it depends what you wanna do with it. I got the Tesco one 'cos it cost next to nothing and I knew that after I'd used one for a time I'd have a better idea of what I want. And I do.