I've tried using one of the cheaper FM transmitters in my car and found it very frustrating to the point of being useless. With the airwaves so crowded it's difficult to get a clear signal that will last long enough to make it worthwhile. From what I've read in similar threads over the past couple years, even the more expensive models aren't worth the trouble. You might have better results depending on your area but if you buy one be sure to save the receipt My current vehicle has a tape deck whack an adapter works fine for. My next one will have a USB port.
I've tried them all, from the cheap to the the expensive. They all suck IMO. My advice would be to get one you like and try it. They all seem to have the same quality. Like I said though, it just my opinion from my experience.