The size for one. The N810 fits into a normal size shirt pocket (might even fit in a pencil protector... haven't seen those in a while).
The keyboard is nice, though a bit awkward, should have a 1 or 2 mm of space at the top. The OS interface for typing still way too kludgy (e.g., predictive typing should be selectable on/off).
The GPS is not that fast, but for me it was quite adequate. Having it in a single unit does make a difference. Having said that, I mostly used it in the car, only a few times while walking (used it for some ag research, and it was quite useful in that setting).
Display worked adequately in sunlight (transflective screens are compromises between reflective and transmissive).
The camera and light sensor are under utilize. Few if any applications use the camera properly, and most any cell phone camera is superior in resolution, led flash, etc. The light sensor... not sure what it is there for, but I used it to read nits and make shade cover calculations (small python cludge). It is not used to turn on/off the led backlight for the keyboard (not sure how that works, as it comes on and off in a rather erratic way), nor is it used to adjust the screen's brightness (another nice idea badly implemented).
Skype works well, so does Fring, but with the recent official support Fring on the iTouch... and the badly implemented BT HID interface on the N810, I have no idea why I would get one again (WIMAX or no WIMAX).

I don't have the N810 anymore, but still have a N800... about the only thing I have it for is to listening to news and as a second Skype machine I keep in the kitchen... thinking about putting a strong magnet on the back and putting it on the fridge's door (as it has become an internet device for my kitchen).