Lacking any experience with Bluetooth, I passed on the option when I orderd my E60. After finding more info on it, I tried to see about retro-fitting it but was disappointed to find it would cost between over a $1000.
So I decided to look at the aftermarket. I first planned to buy a kit that would be permanently installed in the car, but decided against it when I decided to sell my E60 for an E90.
Instead I opted for one of these babies.
Parrot
I found one for about $50 on Ebay and I can tell you it's worth every penny. I was able to pair it with my Nokia 6310i in just a few minutes and its been working like a charm every since. It even allows up to 10 numbers for voice dialing.
Of course it's not integrated like the factory option, but I find it good value for the money and highly recommendable.