I stare at/study people all of the time. I developed this habit whilst living/traveling overseas. Trying to study the "locals".
I am pretty good at being discrete about my staring. I have made it a science. Sunglasses, cell phones and reading material all really make good for good concealment.
I love to watch, study, and appreciate someones style/look/personality. Sometimes it is just interesting to watch people interact with each other. It can be the so entertaining when you really just pay attention to what is going on around you. So often we go on "auto pilot" and block out it all out.
Don't get me wrong, some people really need to be blocked out though, like the annoying twat at the gym who talks on his bluetooth phone headset whilst working out by himself at the gym (the whole time gazing longingly at his own biceps as he lifts weights).
DH and I always point out interesting people we spot. We love to eavesdrop on people's conversations in restaurants too.