The Plantronics Voyager 510 Bluetooth Headset with Multipoint Technology is one of the best reviewed headsets available. I bought mine in May 2008 primarily because of its ability to simultaneously connect to two cellphones.
I still use it today with the NOKIA E71 and the Samsung i780 without any issues. After the initial handshake with both phones, I could initiate or receive calls from either phone and the Voyager automatically determines which one to communicate with.
Why use 2 phones? My line of work requires me to transfer departments every 6 months or so and it is just convenient to keep one with all my personal contacts intact. That way when the time to move on comes, I only have to change the directory of the business phone.
Since I bought the Voyager, several Multi-point Bluetooth headsets have been released in the market. But I stuck with the Voyager simply because of its decent battery life (2 days of my normal use) and flawless handshake with the 2 cellphones.
Just like any headset, the Voyager still makes you look like a geek but it always works and very comfortable to use all day.
* Excellent battery life means very little downtime for charging
* Simple to set up - instantly compatible with numerous devices
* Big, bulky design makes fit difficult with glasses