I'd recommend this...
I've also gone through a variety of FM Transmitters, and have settled on this:
http://www.wholehousefmtransmitter.com/affiliate-referrals.php I have version 1.0 which is not as fully featured. There's an important "trick" you must follow: you can see a gain dial when placing the batteries in. Just crank it clockwise. You can find more detailed instructions with a search.
I mostly use it in my house now to broadcast satellite radio from one receiver (it's stereo - not all fm transmitters are.) It's impressive. I easily get signal about 1/4 mile away. I also have used it in rental trucks when moving (2 cross country moves in 1.5 years.) It's brilliant, I can recall only one "bleed through" in my travels (although it may have been the satellite reception itself.) Changing the host frequency is easy, but unnecessary.
The negatives: (a) it's costly. Probably twice as much as a griffin or other (b) It can create a rat's nest of wires if you use all the features (power wire to unit. wire in from ipod or sat radio, ipod power. There's more with sat radio as there's an antennae connection.) I generally tend to use it on battery power, and cannot remember the last time I replaced any.
I'd highly recommend it if you want the "premier solution" and have the money to spend. My experience is it works better than the other options, and the investment is worth it to me. Other options may meet your personal cost/benefit better, but you won't find a better unit than this one.