I've tried them all and I have to say KDE358 is the best OS replacement for Maemo. Penguinbait has done an excellent job exporting a very usage KDE environment to work with the N810, limited keys, screen, and hardware. Come with many useful programs like KOffice, key mappings, sound, and printing support.

Installation is very easy with just a few clicks after cloning Maemo to the internal memory and a fast SD card. I recommend class 6 or better.

Go to penguinbait.com . Be sure to download the cloning and backup tools first though.