Most people who start these sorts of threads are already sold. No you're not making a mistake.

I have a n800 with the ThinkOutside Stowaway. It's cool, but it has two annoyances:
1. The keyboard Stttutteeerrsssss. It will sometimes decide to transmit multiple keypresses instead of just one.
2. The layout is suboptimal. Placing the uparrow and question mark on the same key, for example, is asking for trouble. Hit a shift-uparrow for a question mark, and let go of the shift before uparrow, and the keyboard decides to move you up a line (or with stutter, several lines). If you're touch-typing, you won't notice that you're now inserting text several lines up.

Also, a built-in thumbboard would be very handy in many situations.

GPS is not an everyday necessity, but I have been surprised by the utility of having a BT GPS unit.
The camera really isn't useful for much yet; video calls are cooler than I thought they'd be, especially since you don't have to be parked in one place to make them.