Thought I would add a review for this thing. Didn't buy it through Amazon but through a local store which had it on the shelf for $10. Figured it wasn't much of a loss if it croaked on me. Caveat to the review: I have not tried the USB or SD inputs yet. I have all my music on my Android phone with a good media player and scrobbling support so that just gets fed to the Aux input on the transmitter. With that said, the whole review and the tips below are focused on the Line input of the device and do not take into account anything to do with the SD or USB ports.
After a few days of playing with it and tweaking out some of the quirks, I find this great as a generic FM transmitter with the bonus of a remote control, digital frequency selection all through the band, and MP3 playback internally. To the quirks though:
1) I found initially that charing my phone and feeding through the transmitter at the same time introudced a lot of noise in the audio. Not ground hum but specifically a digital-like noise. I get this with my notebook through headphones when it is charging but goes away when running purely on battery. Might be ways around this and might not turn up for everyone. I find simply having my phone charged ahead of time and running on battery produces perfectly clean audio. My trips are only around 30 minutes max so not something I worry about.
2) Overmodulation/distortion: Very easy for audio distortion on this device. I guess it doesn't have any built in audio limiting hardware which can either be a good or a bad thing. Good being it is a more direct audio path with no modification. Bad being, as noted, it is easy to distort. And if all of your music is at different volumes, you are going to be constantly tweaking the output volume of your given device at every song change. And with the maximum level this transmitter will accept before distorting on you, you are going to want to keep the volume maxed as much as possible as I found the resulting volume through the radio was MUCH less than regular stations. My advice on this is to search for a program called Mp3gain which will put the volume of all your music at the same level so no constantly volume changing in the car.
Once the quirks are out of the way, it is a surprisingly good experience for the price. Not CD quality, but about 98% there and exceptionally clean with no static once a clean channel is found (I found 87.7 the best in my area. Bottom of the band is usually a good bet) which is surprising given the car in question is one with the antenna off the back (2005 Kia Optima) so getting good reception from a dinky fm transmitter is a bit hard to begin with. There are the occasional pops every so often but they are rare enough to not be a bother. All in all, deserves the 5 stars from me given my usage.
I do recommend though not using the included patch cable and using your own. For some reason it seems like the cheapest one they could include as the slightest jostle even at the headphone jack on the phone would cause scratchiness like you'd get from brushing up against a microphone. It is also too short for most cases. You can pick up one anywhere for pretty damn cheap and good quality if you don't have one somewhere in your house already. Monoprice.com is a good place to look here.