I love all of the extra bits and pieces - books especially, because there's stuff to read. I have most of the published playscripts as well (only missing the musical version of Johnny & The Bomb), plus all diaries except the first one.
I'm sure that if I had had lots of spending money when Clarecraft were making models, I would have bought many more than I did [a few tiny pewter ones, and a Death of Rats].
In fact, the only Discworld-related stuff that hasn't interested me is the stamps... but that's because I've not really been much of a philatelist since I was about 8.