At various times I've used pdaXrom, oz/opie, oz/gpe, and cacko on my 3100. And somewhat embarrassingly I find myself using cacko on the 3100 right now. While all the roms have their merits, cacko seems to just work. And since I need to be productive, cacko seems to work best -- although I sometimes give up a bit in choice of apps and gui toolkits.
As an aside, one of the great things about the Nokia 770 is that a number of apps are and have been ported or written for the Hilden App Framework. Since Nokia is actively supporting the OS, a number of developers are porting their favorite apps. It is more than just compiling because often the user interface must be tweaked to match the Nokia environment. But the results can be amazing. Sometimes I wonder what would have happened if Sharp had continued to actively support the community.