I have no experience as I'm currently first year and in uni halls, but I'm currently looking at private halls for 2nd year. I was supposed to be living with some of my flatmates but that fell through, and although I still have an offer to live in a house in Fallowfield, I don't really think I know the people well enough to live with them, so I'm more inclined to chance halls again. I've done a fair bit of research online and by speaking to people, and the general consensus seems to be to avoid Unite even if you can afford it. MCR Students have a number of places but their website is terrible so I don't know too much about them. I went and viewed Cosmo on Grafton Street today and got some bad vibes. The flats were nice enough if you don't mind the pokiest bathroom you've ever seen, but the whole place seemed a bit dead (which is something I've heard a lot), their Internet was down (which it apparently often is), and they didn't seem very organised. I was more interested in the new Cosmo development on Denmark Road, but I was told they won't even have a show flat up for a few weeks, despite originally claiming they would in January. I dug around some old TSR threads too and found a lot of complaints regarding building works from people who moved into the Grafton Street accommodation when it was new, and I know the one on Denmark Road is being developed in phases, so I've been put off that entirely.
I probably sound like a big moaner by now, but I'm going to try get a viewing at Q3 this week, because I hear they get fully booked quite quickly. It looks really nice on the website, it's near campus, and is apparently quite sociable. In fact, I've only heard good things about it. It's about the same price as Cosmo too.