I was just having this same discussion over email with a friend of mine ... to quote myself:

"I am finding it to be a really useful device ... GPS nav (with the addon Bluetooth GPS module that I bought), voice recorder, ebook reader, mp3 player, video player, gmail checker (from the comfort of bed :] ), SIP VoIP, puzzle games, taking notes ... those are the things that I end up using it for on a regular basis and it works superbly for all of them."

The majority of use is doing quick checks of gmail via the mobile interface:
http://m.gmail.com/

The functionality that I still want:
- BT headset support for voice recorder and VoIP via ...
- BT cellular broadband support ... not Nokia's fault but rather the Canadian Cellular providers

Other than that, it is a brilliant device.