Think this series is a good idea, but really poorly executed. It's an interesting concept, and could have been really great telly. But the writing (or lack there of) is what lets it down.
Although the cast is impressive, it's obvious some of the cast can't work well with the improvisation. And some, if not all of the pairings aren't believable. Vicky McClure and David Tennant didn't work. She is a really subtle actress where as his method is totally over the top acting. Also as others have said, inconsistencies in the stories - like the kids looking too old etc.
It's frustrating that the wives have been so submissive and forgiving, yet more weak portrayals of women on tv.
Think the Billy Piper one could have been good, but the 'love' story was marred by the school teacher / position of power imbalance. Why not just have a lesbian story?
What baffles me is, how could the creators have watched this back and thought that it was decent TV? All the comments on this thread are more insightful and on the money.
It's a shame as British TV is crying out for good drama and this falls short yet again.