When I first used it via the NOW TV box the quality was very poor at times but has improved over the last year.. Now I watch via an Apple TV and its' pretty good now. Also watched a few F1 races live last year on the Apple TV which were very good. The live streams of it on the NOW TV box were painful on a large 55" TV.
I had mentioned it on the NOW TV community forums as well and there are posts from a lot of people complaining. I'm happy now though although not to the quality of Amazon Prime with their 1080P.
I get about 12mb and on my mum's connection should still gets a decent picture on 2.5mb on the NOW TV box but she had a smaller TV