I lived in halls for two years, in a 'mature student' block, which was mainly international postgrads. On the plus side, it was quieter and not as party oriented, but on the minus side it was all international postgrads lol which was kind of lonely. As an undergrad, I was hoping to meet other people in my situation but there were only about three of us in the whole building and as we were in different flats we never really bonded as a group. Also, being the only non-international student was difficult as international students can be a bit cliquey (quite understandably) and so I often felt very left out, despite making huge efforts. They all left after a year which meant that every year you had to kind of 'start again' with friendships. I also found that a lot of the people really weren't that 'mature' - they were MAs and PhDs but they still didn't really know how to live with other people. I'm not one for rows and the constant battles between people in the flat really, really got on my nerves. In that respect, don't go into mature halls thinking that everyone is going to be grown up. They're not, and it just takes one or two idiots in a flat to really spoil the atmosphere. I lived with one girl who constantly left food on the kitchen floor, slammed her door all the time and played loud music all night (it helped her sleep, apparently). I also lived with another girl who moved her boyfriend in, and because they got told off, hid him in her room for four months (yes, seriously). They got asked to leave the halls when they started beating each other up. If you're not an uber-tolerant person, then halls might be really stressful ... I know that I'm so much more relaxed now that I've moved out. I can go home and know that it's all going to be the way I left it, and I don't have to put up with the 'he's done this, she's done that' that I found so grating.
On the plus side, living on campus gave me the real university experience. I was able to get involved with stuff that I probably wouldn't have bothered with if I was off campus. It was cheaper too. Looking back, I did get something from halls but I really wish now that I'd moved out after the first year, rather than living in for another.
If you have the chance to live in then do consider it, but find out exactly what kind of students you're going to be living with. If you're hoping to find people to live with in future years, for example, then you're not going to find that in a hall full of international postgrads ... It really does help to have people around you who know what you're going through both socially and acedemically, so if you get the chance to live with mature undergrads then seriously consider it.