I have a Nokia 6600. It works perfectly under Linux. I can mount the phone using NFS and I can access:
C: - 6mb memory
D: - ram
E: - memory card
Z: - rom
I can also make a GPRS connection using a ppp/chat script and I can remotely control my XMMS from the phone (really neat). I use a Sitecom bluetooth usb dongle and BlueZ.
A little OT: this Nokia has some 3D support and it really works in games. On Symbian you can install apps and games just like on a computer - thats why they call it smartphone
The downside is that it has mono sound. If you want all the features of 6600 + stereo I say go with 6620 (I've heard that there are compatibility issues between the two phones though).