Re:
I've been using a Topeak V12 Panoram for a while. Can't recommend it TBH.
The Topeak needs to be woken up manually, whereas Cateye and the like wake up on their own as soon as the magnet passes the sensor. Also, whenever I turned on my headlight (a simple LED affair with 2 AAA batteries), the Topeak's receiver wouldn't register the sensor's signal anymore.
My next one will probably be a Cateye Velo 8 or something similar. Small, simple, cheap, wired.