oh...you're looking at those types.....
I have the Jabra SP700. That and their Cruiser both get reviewed very well. I've been happy using the SP700 as a speakerphone. The Cruiser is supposed to be better with a dual mic so you sound better to the caller.
All of them (including the T505) are just ok when it comes to the FM transmitter. If you care more about music and less about the speaker phone (ie...not a lot of work/client oriented calls) I would seriously check out the
Kensington LiquidAux. You can do a wired audio connection to your head unit if you have an aux input or you can drop an extra $20 or so and pick up a
wired FM modulator that plugs inline with the FM antenna and gives a way better signal with almost no interference compared to the cheap and not very powerful FM transmitters in the little speakerphone units. The Liquidaux comes with a wireless remote that clips to your steering wheel as well. Check out
this thread at PreCentral for a lot of information on it. And there was also a thread started
here too, but the PreCentral one is quite extensive.