Welcome to BB Forums!
I'll take a shot at these questions, in no particular order:
1) Fun; very little. The BB is a business class device. No camera, no MP3 player, no expandable memory. The hobbyists miss it, but most business people don't.
2) Carrier - the big question. Which carriers work in the people's homes, on their commute and where they work. It is NOT an easy question. You can mix and match carriers. BES is carrier agnostic and doesn't care. We have BBs from every carrier on on BES.
3) Apps; check out the General BlackBerry Forum sticky threads FAQs. There are several "Killer application" threads out there. The amount of free software pales by comparison to the Palms, but somehow we deal with it
4) Bluetooth is extremely limited on a BlackBerry. You can do headsets, as you mentioned, and you can use the BT to sync PIM data in certain circumstances. You can NOT use the BT to act as a modem link, or to beam files around. It is very limited.
Hope that helps!