I have the T-505 and like it a lot. There's a bit of noise on the other caller's end with the iPhone 4, and I'm not sure if that's due to the Bluetooth issues with iOS4, or if it's inherent in the Motorola. But it's livable according to the callers, just mildly irritating. The FM transmitter on it works fairly well, if you need that feature.
Given that I'm not sure where the excess noise is coming from or if it might be fixed with the 4.1 update, I don't think I'd recommend it for heavy call usage. I use mine only occasionally in the car, so it's less of an issue for me than it might be for someone who needs it several hours a day.
I have the Blueant Supertooth3 or ST3. It's the most awesome bt speaker phone I have used. It works really well: loud, clear, simple to set up, and voice dialing works extremely well without any training. It just works.
But the most awesome feature of all is vibration reconnect. This save your battery. I charge mine once a month. Below is the from the manual:
Automatic Vibration Sensor Reconnection:
# Automatic reconnection occurs when the user (without turning OFF the kit) goes out of Bluetooth range for longer than 20 seconds & then goes back into range. When the kit feels vibration (like a car door opening) it will search for and connect to your phone
I think this feature alone tell me a lot about this Blueant company: they really think outside the box to make their products work well with less intervention from the user.
I looked at the S4 model and while it has other really nice features, it doesn't have a vibration sensor. But I wouldn't hesitate to buy a product by them it the listed features satisfy my needs. That vibration sensor is freaking cool though. Maybe they will release a new ST4.
The vibration feature sounds pretty cool but why doesn't anyone make a bluetooth speaker that plugs into a power outlet AND goes on and off when power is detected. My Garmin works this way so I never have to remeber to turn it on/off when I get in my car. Why no one makes a bluetooth speaker that does this is beyond me. Plus I hate having to remember to charge the bluetooth speaker.
I've tried a few different bluetooth speakerphones, including the ones built into various Garmins, and the audio quality has been iffy.
Motorola made a few models that are powered directly, but they recommend professional installation. If you are handy you can do it yourself. That was the route I was looking at as I didn't want to have to deal with the hassle of charging.
The problem is that for it to work well, the microphone has to be more or less in front of you. The mic itself tends to be small, so it's not as difficult to hide. However the speaker part is bigger so it's a challenge to hide. One could use the car sound system as speakers with the right harness, but for my stereo it's a bit complicated due to the "premium" factory sound system. Plus, I didn't want to have to remember to turn on the stereo every time.
Long story short, the best location to attach a handfree speaker in my car is the visor. The Blueant ST3 worked well there. I have to charge it once a month, and it will remind me with a voice when its battery is low up on connecting. It came with a car charger, so if I wish I could run a wire to the visor under the headliner. But it's simple to remove the thing from its magnetic visor clip and plug it in down low, so I am happy for now. Best of all, the ST3 costed me about $60 at the time.