I haven't modified the software on the 770 other than the firmware update. I carry the 770 when i want something I can pocket that will provide the ability to surf the web/check webmail while walking around anywhere there might be free wifi, and to collect rss feeds for reading when I don't have wifi. Oh, and I've used it to look at the photos stored on my phone on a decent screen, which actually makes the camera in the phone much more useful. I've used it with slimserver to stream music. I find the pdf viewer to be too frustrating to use, but that's because I don't actually read the docs all the way through, I skip around a lot. I also think Adobe Reader on my PC is nearly too frustrating to use, so I don't blame Nokia for this.

Summary of likes:
-I rarely have to reboot.
-It really does last several days on stand by.
-It really is "instant on"
-The internal wifi antenna is sensitive
-It connects to wifi networks relatively quickly
-The high pixel density of the screen makes it really readable

I haven't been interested in having it perform video playback functions, although if the 770 could access my NAS, I would do those things too.