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.