I've seen these in stores. They look really neat and also something one could use on the chase. However, I think whenever I'm on the hunt for wifi I'll be using my laptop, so having an extra detector wouldn't do much good. I like how this one displays signal strength. However, its also important to know if the network is open or not, and to do that you would probably need a full fledged wifi adapter with software.
Perhaps it would come in handy while scouting for wifi on foot, on bike, or when you are visiting a mall?