I think people do, whether they say they do or don't. Human nature is to categorise.
I think it's hard to judge someone's true personality based on their music tastes. You get the whole range of fans in every genre. But to an extent, you can probably make some assumptions about their behaviour. For example, you would probably assume someone who's favourite musician is Bach would be more comfortable sitting in their kitchen blasting out Radio 3 or getting all the questions on the music round of the University Challenge than hanging around street corners and alleyways in a gang at 1am. The obvious exception being Alex from A Clockwork Orange, but that's fiction XD
I think you may be able to get an essence of someone's personality from how they respond to the music. I have no idea how accurate it is, but from my personal observation, the people who really feel and love the music, and delve right in so to speak, are the people who also study subjects that are more abstract or arty, are more likely to smoke weed etc etc. This is quite a generalisation, though, and almost certainly not accurate on a holistic level.