Cheap - both their plus point and their biggest downfall.
I was with them fairly issue free for the duration of my contract, but when it came to leave - they wouldn't let me. I rang up at the appropriate time towards the end of my contract to let them know I didn't want to re-new it, and they wouldn't let me. Not only did it take forever on hold to actually get through to someone, but sometimes the line was so bad, or the person couldn't understand me, that I had to hang up and try again. When I said I wanted to leave they just kept saying 'i don't think you do, it's a great deal, you must want to take it'. Anyway, after much arguing, letters sent to 3 HQ and the appropriate governing bodies of mobile networks, I was eventually allowed to not renew my contract. I'd never go with them again - for me, the fact their cheap doesn't out-weigh the fact that I know I couldn't rely on them to be of any use if I ever needed them. I'd also always be worrying about what would happen at the end of my contract again!
I went back to O2 and they've been great in comparison - should tell you all you need to know right there...