If you want a simple one without any installation look at the different visor mounted options. Most will go several weeks without need of a charge. It's nearly invisible and you don't clutter your dash with stick on displays, microphones, etc.
These models work well:
Motorola TZ700 Visor Bluetooth Car Kit
http://www.myblueant.com/products/speakerphones/s4/
(If you use an Android phone the Blue Ant S4 is cool because it has apps that give it some features to make the visor kit even better)
By the way, I own some mobile phone stores and installed h/f kits for years. These visor mounted kits sell very well with very few problems/returns.