How do you add them one at a time?
I have had very good luck with my Moto RAZR V3 using Bluetooth in the 400h. The pairing goes smoothly and it easily connects every time I start the car. The sound system works great with no noticeable delay. It's so sleek that I can just leave it in my pocket and use the Nav interface for everything. I also successfully transferred my phonebook to the 400h (although you need to do it one at a time). You can then put 17 names on the 1-Dial feature. These are the only numbers you can dial when moving. It works well and is easy to set up. So far so good.
I also used the phone over in Europe a couple weeks ago and it worked flawlessly. Coverage on Cingular should get even better as they consolidate AT&T towers.
I am using the Moto HS850 Blutooth headset and it works very well. I haven't experienced any significant interference. I wear it around my neck on a lanyard while traveling and place it on my ear when a call comes in.
Good luck.