They are illegal to use in the UK, judging by the web site. You are not
allowed to run an unlicensed transmitter on FM broadcast frequencies, no
matter how low the power. We had this out on the PocketGPS forums a
while back (people were using FM modulators that radiated in free space
with their Pocket PCs). They're legal in the United States, so far as I
know - but not over here.

Further, it looks like a really bad solution anyway - as you can't use
your radio for anything other than the hands free device. I've also had
very bad results with anything that clips over the microphone and
speaker.

"Proper" hands free kits are not expensive now.

David