I have a lot of experiance with the 770. I find the PDF ability to be usable but not great. It may just be that the PDFs I tried to view were not well formatted.
As for other features I don't use it for the internet as much as I thought I would when I bought it. I use FBReader
a lot and also Simple Center
for streaming audio from my computer.
I have also used the VNC Viewer
which worked well for keeping me on the couch instead of in front of the computer.
I also play a ton of Mahjong on it since I haven't found a version of Freecell for it.
I am going to be putting it aside as soon as I can get the Nokia N810 though since I got a chacne to play with it the other day and fell in lust with it.