I lived in Barnes (S Block) last year and it does depend on which of the blocks you're in. A-F are the taller blocks of individual rooms (with sinks), G-L are the taller blocks with shared flats, N-V are two storey blocks with shared flats and X/W are shared flats with en-suite rooms.
I've been in rooms of each type of block except for X/W. A-F are decent sized rooms, obviously having your own sink is a plus although the kitchens can be a nightmare because you do share with a large amount of people. In the shared flat blocks the kitchen situation is a lot nicer as you're only sharing with 3 other people so you have heaps of cupboard and fridge space.
I honestly had a great time living in Barnes last year but that may be done to living with friends who I'd known for 2 years previously as I lived in Barnes for my final year. Saying that if you do get on more with people you meet outside your living situation then all the other halls are only a few minutes walk away with the exception of Hawthorns which will take just a tad longer to walk to. Anymore questions about the amenities in Barnes or the accommodation itself then I'll do my best to help!