Nokia kindly gave me a developer discount code for the N800 internet tablet, a few weeks ago. That was a very pleasant surprise. Actually buying the thing was complicated, to put it politely. I finally laid my hands on the device his Monday. Yum, yum!
Changes I like best: extra RAM, speed, storage space. The built-in stand. Position of the headphone and charget sockets. Screen (shinier, not as grainy, although I think it reflects a bit more ambient light than the 770 used to). The ability to reorder status bar icons. New Opera toolbar. Backup application that works without killing all other applications and entering offline mode. New themes. Battery time estimates.
Changes I'm not sure I like: new stylus (too short). Lack of hard case (without it I'm forced to lock the keys when I stuff it into my pocket, but the tiny power button is hard to press). The new shape (it's harder to hold it in my right hand while pressing the down button, which is how I like to read books sometimes). The tearing effects when panning in Opera.
The new Media Player merits a category on its own. It indexes all the media files (songs and videos) in ~/MyDocs and in all memory cards automatically. That's very nice when it works. It's frustrating when it doesn't. There's no way to force reindexing after you shuffle files around manually, and no indication when the automatic reindexing is finished. You just have to wait and hope that it will catch up. Also, sometimes the user interaction is very strange: bug 1056, bug 1063.
Overall, I like it better than the 770.