I am in school, but I don't use my notebooks for class.
I've actually got a couple I write in frequently.
In my grad work, I find a lab notebook comes in handy. I essentially write down everything I think of, and everything I do in the lab. Keeps a record of the work I've done/plan to do, which can be important.
I keep a daily journal just to vent about daily bullshit.
I carry a small one in my back pocket pretty much all the time, to jot down random crap. Phone #'s, email addresses, title of a book I want to read, lists of stuff I need to do, whatever.
I fill up a lab book every few months, journal about every 6, and the pocket book never fills up, they just fall apart after a while, I never fill them up.
Here's a question, does anyone know of a better alternative for a back pocket book than the moleskine? (specifically, these guys: link
The hardbound ones are too thick (200 pages), but these aren't tough enough.