I haven't used one with my scanners but I did give a Bluetooth transmitter a try on one of my guitar amps. I believe it's a Taotronics branded transmitter, an older model. Suffice it to say there was about a 1-2 second lag between playing a note and hearing it through my Bluetooth earbuds. Paired a set of LG Bluetooth earbuds with it so it wasn't paired with a receiver, which would result in extra lag. It shouldn't be an issue with a scanner if you don't mind the lag.
What "system" does your car use? Some are better than others. I have Ford Sync (non touch screen), it sucks. It's already laggy and cuts out momentarily with a straight Bluetooth connection with my phone when I want to listen to music. I just have my scanner hooked to a ground loop isolator (use a power inverter for my Pro-197) and then to the AUX port. Works fantastic.
I signed up for amzreviewtrader (figure out the URL) and managed to get a pair of Aukey Bluetooth transmitters/receivers for 50% off, among a lot of other things (Android TV boxes, Bluetooth speakers, etc). It's worth taking a look. They've got some cool stuff there.