The OS is Maemo 5 "Fremantle", which is based on Debian (and BusyBox), but some of the ways it's set up aren't fully compatible with a lot of Debian standard software. I don't think you can just add the Debian ARM repositories directly and install stuff. Packages have to be tested and sometimes modified to work natively.
However, it is popular to create a real Debian environment with chroot, which works around that problem. See Easy Debian
, which is a package that does all the work in setting that up, including OpenOffice.org, GIMP, LXDE and an environment you really can apt-get install most anything from the Debian mainline into.
I've had a N900 since December. I'm very happy with it. Installed Easy Debian and OpenOffice and they work quite well. There's only so much word processing I want to do on a platform that size, but it's great for modifying office email attachments on occasion. Having a spreadsheet in my pocket is quite handy too. Stylus is recommended but not compulsory. It's still in testing and there are a few headaches, like some dialogs being too tall to properly reach the buttons at the bottom, but it's already improved a lot from previous versions and I expect it to get even better.
As for the community, the main forums
don't look dead to me. Have a look
at the packages they offer.