So, I'm using my N800 daily, doing chat, browsing, playing media, as an alarm and I can say: it's a great improvement over 770.
Opera browser, the main application, is much faster and reliable. Just now I can use Gmail and other web applications without problems. Flash is still bad for youtube, but suck less for sure.
Thumb mode everywhere is cool, but could be better used, mainly on application menus, where it should be wider, possible taking the fullscreen (since you can't do much while menu is open), more GUI issues were pointed in a nice review by Sean Luke.
Media player is much improved and new possibilities are open due split of Gstreamer elements that were codecs and were outputing video using DSP and now are just codecs, allowing you to use Xv or X11, enabling On-Screen-Display, for example.
However, not everything is good: battery could last longer and video conference could be easier to setup with non-n800 peers and use Jabber/Gtalk instead of nokias own servers, that register your "screen name" with a weird number/hash at call.tableteer.nokia.com. Also bad is the (d)effect that you get from a faster processor and lack of sync between applications and X/video-driver: sometimes you redraw the screen while driver is updating it, then you may have half top on the previous animation frame, while the bottom on the next frame, giving an weird result. This happens in every application, more noticeable on horizontal moves, from SDL games to Opera browser (try drag and move pages). Most of issues are software-related and may go away with software upgrades.
Summary: upgrade ASAP.