I have tried using In-Ear type Bluetooth Handsfree devices only sticking in your ear, always fall out.
I use over-the-ear , with glasses! , they stay in but sometimes easily shift/not always comfortable.
I have used Plantronics Voyager Pro, batteries failed after a year, got new Voyager Pro+, now the batteries are failing with these too!
The sound for me is good, but sound from me is not so good for the other listener when I am driving.
The microphone is facing forward, I am driving a small micro-van, noise from the wheels comes into the microphone and makes it difficult for the other part in the conversation to hear my voice clear!
The best solution is the Nokia BH-214, and if you do not like to plug both your ears , use only one plug!
The receiver is clipped to my shirt, the microphone is pointing up and from forward and back towards my face, does not pick up engine or wheel noise.
My phone is placed in a holder on my dashboard, to comply with Police regulations to use Hands-Free cell phone while driving.
There are other similar devices like this available , I tried also Sony Ericsson MW-600, but the sound from this was awful!
The Nokia BH-214 can be found cheap on eBay, but beware! They are copies!
These obviously does have different electronics!, and some have had problems with connecting to new phones with newer versions of Bluetooth transmitters!
So you will have to pay 50 USD or more to get a real Nokia.
Nokia have new versions of this type of Bluetooth devices, look in eBay or Google them, you even get with radio!
This is my best recommendation for using while driving,