You can install the EasyDebian from extras and let it download the image file automatically (or download it yourself from a PC and transfer it over it's fast). Note that if you let it download the image file with the N900, it's slow. (see easy debian thread for more details)

As for speed and usability.

Speed wise it's slow to start up. And you can't multi-task alot with it. I can usually only run a browser, IM, and OpenOffice (and even then it's starting to go into the slow realm). Usable, it's mostly usable. Writer is fine. Impress (Powerpoint) isn't really usable since the various panes take up so much room. I find that it's more useful for editing existing Powerpoints than making a Powerpoint.

Saving is also slow.