I'm an Eris user, but the design for the Eris and N1 is a little similar, and I use mine a bit..but I used to have a Sidekick LX, so I like trackballs. And I love color notations.
I don't use it that much, but I like to select links with it. You can also swap the homescreen pages with it but I usually just swipe.
And, I HAVE to use it to take a picture, which is lame.
Oh, but I DO use it for editing text in a text message or whatever. Pressing in the text box and hoping it lands in the place you want it is difficult.