While on hardware wise it doesn't seem worth upgrading, I have found the slightly smaller size, transflective screen and better pouch a nice "not so obvious" upgrades from N800.

The downside in your case is the single mini-SD card slot in N810 that will restrict the available storage space currently to 16+2GB. For N800 you can easily get 32+32GB SD cards if so desired.

For me the GPS has been satisfactory. The keyboard is also pretty good for short text entries, and especially when doing keyboard shortcuts (CTRL+C, CTRL+V, etc...).