Was looking for an iPod transmitter for our car journey to France when I came across this; wasn't sure what to expect for less than £6 but was greatly impressed!
Simple to use, just find an unused frequency on the car radio, tune the transmitter to the same frequency and that's it!
No humming or other interference as others have commented - certainly none that was noticeable over the usual road noise, sound quality is comparable to a good FM radio station reception.
Occasional interference was caused by other radio stations coming in on the same frequency - inevitable if you are travelling any distance - which required the transmitter to be re-tuned - but other than that, this is a great little gadget that simply does what it's supposed to.
My single regret is that I didn't spot the version with the remote control which cost about £1 more than this version...