As a long-time 770 and N800 user, I finally bought a N810. There have been various reviews of the N810, so I will concentrate on the points which I did not see anywhere else, or on the points where I do not agree with the previous reviews.

First: the GPS. I read a lot of comments about poor GPS performance. I did a few tests over 2 days, and I can't confirm these reports. GPS performance seems quite comparable to other devices to me. FIrst cold start in about 10 minutes. Restart after half a day is a few minutes, depending on sky visibility. Warm start is under a minute. The times seem comparable to other devices I have: Holux GPSSlim 240 (bluetooth, SIRF III) and my old Garmin iQue. Sensitivity inside buildings is closer to the one of the Holux (good: SIRF III), than the one of the Garmin (poor: older chipset).


Second: the screen. The N800 could be almost impossible to read in sunlight (e.g. driving in a car... bad for GPS applications). The N810 has a transflexive screen. The good news: it is readable in a car in a sunny day. The bad news: but only with the light full on. The "transflexive" part is noticeably less efficient than on older devices (e.g. the Garmin iQue, or a Nintendo DS), I suspect because of the high screen resolution (smaller pixels mean less light will get in). Colors are also washed out in the sun. Still: it is a noticeable and welcomed improvement.
A small note: I notice fingerprints more on the new screen, I am not sure why. I tend to clean the screen more often.

Third: the keyboard. For me, this changes completely the way I use the device. Not having the keyboard pop up on the screen leaves much more screen real estate. On the minus side: to use the keyboard it is much more convenient to put the device on a table. It is not so convenient to use it freehand. About other keys (on top and on the side): the "lock" switch is a welcomed addition, I am still not very sure that the keys on the front are well chosen (I would have loved a menu key on the front, for example).
The keyboard is lit. Very nice.

Stylus: much smaller (like a palm stylus) and less comfortable to use.

Sound: the N800 had great sound with surprisingly good stereo separation. The N810 is much worse. Still OK for a device that size, though. On a side note: neither the N800 nor the N810 are loud enough for me to hear driving instructions on the motorway. My car is a bit loud, but still...

Case and pouch: the N810 looks much nicer than the N800, in my opinion. I think that the pictures do not do it justice, it looks much nicer in reality. The N810 in its (thin) pouch is also much smaller than the N800 in its (thick) pouch, which is important for a small device, IMO. OTOH, the case scratches much more easily on the N810 than on the N800 and the new pouch gives even less protection. The new built-in stand is flimsy.

Camera performance appears unchanged.