Looking back one year, what I am spending most of the time with the 770 is: reading text and ebooks. Fo that purpose, it's even better than the 800 due to the integrated cover and the automatic stand-by mode when sliding the cover over it.
So: I'm happy with the 770, the 800 would even be a step back (cover issue)

What I might use the 800 for: due to the built-in stand, it could be a nice always-on picture / movie frame or. However, for that purpose it would need for aestetic perfection:
- a second power plug at the back - I would not want a cable coming out at the right side of a picture frame.
- the buttons would be disturbing
- ideally: extremely low power comsumption, so that I could use a solar power panel to power it it on permanently.

Im not sure if Nokia inted the device for this very use.

However, I remember they once sold a "SU-7" image frame. It wasn't much cheaper when I last saw it, around €350 or a little less. I don't know if it's still produced.