Southlands halls are ok, not that old. Epworth and Aldersgate are the halls based around the central quad, and I've been in a few of the rooms. They're not too bad, although they can be a bit tiny - things like the bed sort of goes under the desk in some of the rooms, and there's not much in the way of floor space, that sort of thing. Some of the en-suites are tiny too, they look a bit like an aeroplane loo with a shower lol. However they're very central and you're right on campus, which is handy if you're likely to be having a lot of lessons at Southlands. I've heard though that they do have quite a good atmosphere. A few of the rooms were a bit 'tatty' when I was there, but I think that's fairly typical of most student accommodation.
The nicest halls on campus are either Lee House at Digby, or any of the Whitelands Halls. They're all fairly new (about three/four years old now) and are en-suite self-catered. The rooms at Whitelands are all the same size (unless you luck out and get one of the absolutely mahoosive disabled-access rooms), not huge but spacious enough. The en-suites are nice too, quite big 'wet-room' type affairs, and the rooms have a fair bit of cupboard and shelf space too. The benefit of the Whitelands accommodation over the Southlands is that the Whitelands was designed with quite large kitchen/lounge rooms. At Southlands you've generally just got a small kitchen and there's no real 'social' area - if I did hear any complaints about Southlands it usually was that the kitchens were too small.
The main problem with Whitelands is that it's not on the main campus, and although it's not a long walk it can get fairly tedious walking up and down the hill. Also, people do tend to choose their accommodation based on their main 'teaching' college, so if you're an education student, for example, you might not have many fellow students at Whitelands as it's mainly psychology and science people. It's not a massive issue but obviously if your friends/coursemates are based on the main campus you might find the travelling between the two really annoying. However, at Whitelands you do get the benefit of some of the better accommodation and one of the nicer bars. The other advantage to Whitelands is that you're nearer to the shops of Roehampton, which means less of a walk to get day-to-day stuff. However, the area isn't brilliant. Whitelands is just on the edge of a council estate. It's not *that* bad an area (I now live in SE London and Roehampton seems like a paradise lol) but you do need to be sensible.