Use it between my cellphone/PDA (nice to look up someone's info on the PDA, tap their number and have it dial them for you on the phone; just wish it would properly auto-sync between them), and cellphone/PC (lousy Nokia 3650 doesn't even HAVE a serial sync cable; you have to use bluetooth). I also have a Jabra Freespeak bluetooth headset for my phone (very nice, especially while driving). I suppose I could also use it to sync up my PDA to my computer, but I usually drop it in the cradle to recharge anyway, so I never bother. I tried 'net access on my PDA through the phone, but my service is usually too flaky for GPRS to maintain a high data rate.
The only issues I have are that my phone isn't smart enough to be able to talk to the headset and the PDA at the same time, and that the phone is also not smart enough to sync address books up automatically with my Palm Pilot. But since I mostly use the headset while driving (and I don't usually use the PDA while driving), and there's third-party software that can keep my address book intact, it's not *that* big a deal. Definitely a bit of a hassle, though.
There are also still bugs to work out in terms of hardware compatibility. The first USB bluetooth adapter I bought didn't work with my phone (I didn't know they MADE ones that didn't support the serial port emulation profile! What idiot thought that up?), and the Jabra headset I bought turned out to be an older rev that also didn't work with my phone (Jabra blames Nokia for switching hardware at the last minute, and replaced it for free). Nokia's sync software is also a bitch to set up and use, and is nowhere near as smooth as what Palm has. Considering the whole point of Bluetooth is supposed to be seamless compatability between devices, this is not exactly promising.