In reply to Sib81:
I picked up one for €8 from my local supermarket - it does all the normal speed/distance things, plus calories (although that is clearly at best a guess), and temperature (which seems pretty accurate). It's small, easy to use and seems very reliable after nearly 3000km.
To be honest, as long as it picks up every time the wheel turns, then the "accuracy" is mostly down to you. Accurate timing is easy and cheap with moderate electronics (for exmple, the clock on mine has slipped by about a minute over the last year, so that's acurate to within 0.0002%) and then all it needs to do is count how many time the wheel turns - it can't get that wrong! Where you could potentially make a mistake is measuring the circumference of your wheel - be careful about that and you'll be fine.
Unless you're swinging by Germany soon, I can't really help you with where you might find the same one, but the point I'm making is, it doesn't have to be super expensive, unless you're looking for features such as altimeter, heart rate, cadence, GPS and being able to download it onto your computer etc.
Tim