My wife has one. We bought it before a 2-up trip last summer to Yellowstone and Glacier. Initially I was thinking gimmick but after picking off 5 parks/monuments I actually like that she's interested in stamping the book. It keeps her interested and engaged as we ride around and it took us to places we might have otherwise passed by, just to hit the visitor center for the stamp. We saw some cool stuff on those little side trips. A couple things to know:
1. There are usually multiple places for stamps in any given park or monument. Yellowstone had 4 or 5, maybe more. Do as much or as little as you want in terms of hitting the spots.
2. They sell stickers to put in the book to show an image of the particular place. Otherwise, it's just blank pages with stamps, exactly like a normal travel passport. Yeah, my wife is all about the stickers too so that adds to the cost. Whatev, she's happy and out there with me on the 990...I'm happy too. Small price to pay!
I think it's worth the price of admission.