Big problem with most cordless cycle computers is that they are very directional, so only work if the computer is directly above the sensor.
I was experimenting with this the other day, the standard cateye cordless has a range of only about 20cm if not aligned...but can be as much as 1m if aligned right. Only problem is you can't get a good alignment between your rear wheel and anywhere useful.

Cateye has a new(well relatively new) cordless computer which does speed and cadence, both measured from one sensor next to the rear wheel/crank arm. Its supposed to work very well but I haven't used one yet. Also gives the benefit of cadence...which in my opinion is one of the most important features on any cycle computer.

I know others on this forum use them.

I have the Cateye Double Wireless to which Triguy refers. Very happy. Works great so far. Works on the trainer, etc, AND it keeps lap statistics. It has a single unit on the rear that has a sensor on one side for the crank and the other for the wheel.

Hey Monk, I bought the Cateye Double Wireless for my daughter's new bike. My husband put it on his bike before her bike was done and he loves it. Now that her bike is built, I doubt it would have worked on her frame, there is less than 2cm between her crank arm and the frame(48cm). Any suggestions on what would work under those tight conditions? She definitely needs cadence, don't really care about too much else. Thanks!