I'm a mature student who has stayed in halls for two years ... and I can't wait to leave lol. It's ok for the first year, as yes, it is a good way of getting to know people, but I think that I was slightly unluckly in that at my university, they put the mature undergrads and the postgrads together.
For the past two years I've been the only undergrad in the flats, and although I've made friends it's difficult because every year the MA students leave and you have to start from fresh again. Also, it can be difficult because the vast majority of MA students (at my uni anyway) are international, and they don't really mix, which can make life in halls a little lonely.