I first bought a pack of Field Notes because they looked cool, but I missed the entire point of how to use them. They're sized to fit in your back pocket. I now carry one with me all the time. It is the perfect thing for the "fourth pocket": iPhone, keys & change, wallet, and Field Notes.
There's something about watching people who love their craft that is really engaging. What a gorgeous process, and a well done short film by Coudal.