Technorati Tags: travel
I always wear one when I fly, because it's got fantastic pockets. The zippered one is exactly the right size for my passport, while the Napoleon pockets are good for tickets and notebooks.
They can also hold maps with ease (though I confess I tend to just get a subway map, and otherwise bomb around wherever I'm at). You can even get guidebooks in the pockets, if you're going somewhere relatively uninteresting.
They're also the Unofficial Shirt of Bay Area Futurists.
Both Stewart Brand and Paul Saffo, two guys who have pretty exquisite taste, rave about them on Kevin Kelly's cool tools blog.
Curiously, so far as I can recall, I discovered them completely independently of either Stewart or Paul: I was in the Royal Robbins store in Berkeley one afternoon, and just stumbled upon them.