My spoke magnet works fine. I don't know the brand or anything about it -- a mechanic at the LBS put it on the bike for me, probably when I first bought the bike and they were setting up my old Polar cycle computer for me.

I do have problems with the speed sensor on my mountain bike not working, even when it is hitting the magnet as the wheel spins. That bike has a low-end Bontrager computer on it. I haven't yet tried to figure out what the problem is with it.