WE have just started watching it after reading rave reviews. We have almost finished series one. (I do like having the whole boxed set and not having to wait a week to get the next episode. We have a lot to catch up with!)
Reviewers seem to agree that the series ran out of steam by series 4 and wasn't as good. I can't comment because I haven't got that far yet! We are enjoying it. It's a darn sight easier to follow than Line of Duty, which has just finished, but it's equally as exciting.
I think some of the sex scenes/stripping off to underwear scenes are a bit gratuitous though. My only reservation.