Having played around with an N810 for a while, there are definitely pros and cons compared to an N800.

The smaller size is great. Unfortunately, I seem to end up with the keyboard open almost all the time for keyboard and Menu key access, which makes it actually LARGER than the N800. Even with the keyboard out, I still end up with a stylus in one hand because not everything is keyboard accessible.

I also put my thumb right on the light sensor. Constantly. Not on purpose.

The 810 has a much nicer looking casing. And the hardware keyboard is quite handy.

And it doesn't have the camel hump on the back that makes the N800 so uncomfortable to hold.

But the placement of the D-Pad and the Menu key is a huuuge bummer. It's extremely inconvenient. If you already have an N800, the value of upgrading is debatable.