That page of Sebastian's hasn't been updated for some time. For the current status, please check the N900 kernel status page. The n900-modem driver was merged in 3.16. If I'm honest, I haven't actually tested the modem yet.

I've only really tried Enlightenment which has an option to optimise for mobile devices. It works quite well but has its quirks such as that it ignores all X11 keyboard settings that have been set up in /etc/default/keyboard, /etc/X11/xorg.conf and /etc/X11/xorg.conf/* so it can only really be configured via the command line or with a not so elegant hack. It's easy enough to install (apt-get install e17) but unfortunately the package maintainers are a bit behind. E19 has recently been released.

I would like to build Plasma Active at some stage to see how that runs. There's also a package for Plasma Netbook available in Debian. Aapo is working on porting Hildon to GTK3 to run under Debian. Some prefer more basic window managers. There are a few options so I haven't yet decided on which UI would be best to install by default therefore my scripts don't install one yet. X11 is installed and configured for the keyboard, touch screen and I'll also configure it with the SGX drivers when I get that working. I welcome contributions of separate scripts to configure specific UIs.