Looking to buy a Garmin Nuvi, but I'm not sure if I need the bluetooth option. What will bluetooth do for me on a Nav unit?

Off the top of my head:

You can sync it with a bluetooth phone for hands free operation through the Garmin's speaker.
You can send addresses to the Garmin from a phone or PDA instead of doing it manually.

I say it's useless if you already have bluetooth option on your E60. It doesn't offer anything like A2DP so it's just another hands free calling feature. I'm surprised it's not even a standard for lower end models because bluetooth is SO DAMN CHEAP. Only in the U.S., I believe bluetooth is even consider a feature nowadays. Bluetooth is very insecure and outdated really. You should also know that speakers in any small handheld device is crap. Only get bluetooth with at least A2DP features.