I originally bought series 1 to 3 and was impressed enough to buy all 22 episodes. The scripting and acting is excellent, and the personalities and frailties of the main characters are very well developed in each succeeding series. I found myself almost screaming at Detective Inspector Mike Walker (David Hayman) because he appears to lose sight of everything going on around him, except for the case at hand. His personal life is a train wreck and elements of this is trawled through in some of the earlier series. DCI Roisin Connor (Victoria Smurfit) is Mike's female co-star, and she's no shrinking violet either. She's happier to kick the door in than ring the bell, such is her eagerness to sort it. The original "Wonder Woman" on steroids, she seems to hide unrequited needs not fully revealed, which makes for a level of mystique in this capable but highly complex woman. And then there's DS Satchwell (Dorian Lough), the hapless underachiever who seems to coast along under Mike's shadow, doggedly doing all the donkey work, without finess, depicting the average foot soldier commonly seen in most jobs.
David Hayman and Victoria Smurfit are excellent choices for their respective leading roles, giving convincing portrayals or some seriously flawed coppers. I consider this to be a first rate series.