I bought a Sony BT2500 off ebay for about $100, replaced my stock Nissan stereo in my car with it. Not only does the sony have superior sound and an aux port, but it supports BT stereo as well.
That means I put shuirim on my phone, listen with headphones, then when I get into my car, I hit connect on the stereo, and BAM! Audio through the car speakers.
A2DP Bluetooth quality is way better than FM transmitters, and very often indiscernible from a wired connection if done right. Plus, I keep my phone in my jacket and control everything from the dashboard (as long as your phone supports AVRCP bluetooth remote controls- sorry iPhone!).
I liked it so much, I bought a second BT2500 for my wife's car and installed it myself (had to get a mounting bracket adaptor because it wasn't a standard DIN size, but wasn't difficult at all to do!)
