They work <OK>, a bit of crackle and hiss, and they're quite power hungry, I have 2, one I use plugged into my computer so I can 'listen again' all over the house, the other was in the car, but I've now got a proper solution for the new car. You need to be aware that they have a range of over 1/4 of a mile, and can interfere with regular broadcasts. They're also not strictly legal.
If you've got a VW / Audi / Skoda etc I can recommend the VAGX1 (just google it)
The other car has a tape adaptor thingy which woeks very well indeed.