I got my N810 a bit over a month ago now and been using it daily. Ever since I got it, it has been very stable, no need for any reboots or anything in a while. Uptime is a nice 31 days now.
Look and feel
The look and feel is very good, I like the brushed metal front. It also feels very solid. Speed is very good, interface is much more responsive than on the old 770, the extra 200 or so Mhz really helps, as does the double amount of memory.
GPS works ok, only the initial fix took it's time but that's expected. After that it been working just fine.
The default Map program is pretty basic. And unfortunately to get any proper routing out of it you need to pay, and then it would still be quite useless as you can't save your track. (For openstreetmap purposes). But Mapper(or maemo-mapper) is very good replacement.
What else.. the screen is as good as on the 770, no change there, touch feel is a bit different but not that much. The default theme is ok, but I don't understand why every OS release need to have different themes, I liked old ones and I would prefer to use one of them still.
Skype is now supported and Gizmo should support video calls. I haven't tested any of those yet as almost nobody I know uses any of them that much, almost all are on MSN only. Pidgin is ported so I can chat with friends, would be nice to have pidgin as a default install. And now with the beta update of rfcomm it's possible to use other protocols from the integrated IM framework. Been using MSN with it for a while and it work nicely.
I just tested a video call with Gizmo on windows (Version 4 is no longer a beta, yay!), works quite nicely. Video quality is a bit bad but ok.
The browser is no longer Opera but instead Mozilla based and now heavy ajax sites like Facebook finaly work ok (same with the latest OS2007). And such useless things like youtube works with the new flash.
The new transparent desktop applets look very good, but they take much more space now and the RSS reader won't display a preview anymore, just the title. Also the speed contact only displays as many as can fit, for me that's only two, not very nice.
The interface is now much more thumb friendly, it's pretty ok, but not consistent with all apps and sometimes the interface feels a bit cramped with BIG buttons. I would have liked to have the OS2007 adaptive style for the main menus back.
The keyboard is ok, as good as one can get in such a small space. Some special characters are in odd places but otherwise pretty ok. But I miss the cursor pad on the side as I'm used to the 770. Some say they have a problem with using the top row but I haven't noticed anything, must be my fingers or that I'm typing with the tip of my nails as I'm used to on my N73 with it's small buttons.
Anyway, I'm happy with it :)