I have a Topeak Panoram V12 in a box somewhere. I bought it because I wanted a fancy wireless speedo at a good price. Avoid for 4 reasons :
1 ) You have to manually wake it up (Cateye etc go out of sleep as soon as the magnet passes the sensor), otherwise it won't register anything.
2 ) The controls are bad. Buttons don't do what you expect them to do, weird combinations, you need to learn a lot of those to do simple tasks like resetting trip meters etc.
3 ) If you have any LED or Halogen light, forget about reading your speed when it's dusk or dark. The second you turn on the light, the main unit doesn't receive the sensor signals anymore
4 ) The distance between sensor and main unit needs to be less than 45cm (1.5ft), which doesn't leave you with to many options even with just 100mm of travel. Forget about nicely hiding it somewhere near the axle, On my Schmolke I had to mount the sensor right near the rims to get a decent signal.
As for wireless in general : Keep in mind that with most models you'll need to check the magnet's position every single time you park the bike somewhere.
If it's too close to the sensor, the latter will pick up the magnetic field and transmit a continuus signal. That can drain the sensor's batteries pretty quickly.
As for me, I'm currently looking for retro wired computers, preferrably NOS Cateye Mity2 or Mity3. You just can't beat those things for simplicity and reliability.