I picked it up the summer after I visited the Grand Canyon and Zion (of course). While it's fun to stamp my passport at each new park I visit, it's also frustrating, as I inevitably forget to bring it along about half the time! It is a great reminder of exactly when you visited each park, as the cancellation stamps have dates on them and are color-coded by region.
For example, I'd completely forgotten how much I traveled in 2002. I managed to get four stamps (from four different regions) all in one summer!
I'd love to get back to Acadia and see it without several feet of snow on the ground. Cadillac Mountain was one of the few bare spots that spring break. On the plus side, no crowds...
On our Michigan road trip, I made getting stamps from all four National Park sites a priority. Later, I found out they added another that summer. Oh well.
If I forget my passport, I'll stamp whatever I have on me and stick or staple it in later. Apparently I was carrying post-its around Florida.
The passport comes with a map of the National Park Service's properties (whether they're parks, seashores, monuments, trails, etc.). It's nice for seeing where you've been, locating parks near you, or plotting future trips.