I have the car kit.
It's ok. Not great, not bad. There's a site (belkin?!?!?) I don't recall the URL where you can type in your zip code and it gives you the best channels to use. I did it, and it helped. Here's what I've found in my 3 cars.
05 Mustang - Stock radio - Works great. very rarely get an bleed thru from something broadcasting on the channel I use. No static.
95 Mustang - Aftermarket radio - Works, but kind of sucks. There's an engine speed related static I get when listening to it. I don't get the static when playing a CD in that head unit. If I recall the static is there on the radio to. It's probably a bad ground to the head unit. I also seem to get more bleed thru from something broadcasting on the same channel. I don't use the transmitter in this car much as a result.
97 Expedition - Aftermarket radio - Middle of the road. No static, but more bleed thru than the 05 Mustang, but less than the 95. I use it in the expy quite a bit. (as well as the 05).
One thing that I've noticed helps a TON is to drape the power cord over the rearveiw mirror (it doubles as the broadcast antenna) so it's not all looped up. Definately helps the signal strength.
Having said that - I have an aftermarket nav unit going in the 05 with direct input, and I'll probably put new head units in the other cars with direct input as well (when I get a chance, it's not a huge priority, just something I'd like to do)