I've used one by Radio Shack and I like it. It has four transmit frequencies, two low and two high and I have had to change the "station" as I travel down the road into to the broadcast area of a channel that was previously free. I have never been in a location where all four channels were already occupied. Although, that would certainly increase my chances of hearing something interesting on the radio
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One important consideration regarding the wires. The unit itself is sleek and tidy, about the size of a couple of Zippo lighters side by side. It has two wires, one thin one about 2 feet long that is called the antenna. The other wire is realllllly long, maybe 6-8 feet, and it's the line in cable. There is no power cable, mine is powered by two AAA batteries. I have found that the reception is moderately good at best when the antenna cable is deployed, and the line in cable is bundled up. But when I stretch out the line in cable too, the quality and strength of the radio signal goes waay up. So I wind up putting the cd player in the passenger seat, line out/headphone jack to the fm transmitter and then toss the transmitter in the back seat. The wire trailing behind "deploys" the "antenna" for best effect. Works great. Easily worth the $25-30.