I think the only things that could make xournal better are:
1) Ability to select and move/resize text (similar to how windows vista/7 tablet pcs work)
2) A way to leave the shape recognizer on. For some reason it keeps on turning off while I write.
(note: I actually do use xournal on an n810 to take notes in class)

but other than that, I love it. I can take notes pretty decently on the n810 screen, but I don't know how well I would be able to write on the smaller screen of the n900.