James Nesbitt makes an outstanding performance in this two series DVD set. Each series is a three part story. They make for gripping but unpleasant and disturbing viewing as it deals with issues that most of us can count ourselves as fortunate not to see. It takes crime depiction to another level of detail and some of the scenes make for tense viewing.
Murphy has extremely difficult infiltration roles to conduct and ends up not only up against the very worst of criminals but inter-departmental politics within his own management structure. This ultimately leaves Murphy as a heavily flawed but very likeable maverick.
Watching late in 2011 in both senses, I found this series too disturbing to go to sleep on and so choose series 3 of Doc Martin as some light relief. This is somewhat ironic because these two programmes were transmitted on rival channels at the same time, and had a sufficient impact on the viewers of Murphy's Law for it not to return for a sixth series. This is in no way a reflection of the quality of the series but merely an "at-the-moment" preference over the two extremes on the drama scale. However, series 5's ending is left open to interpretation; whatever Murphy did next would be totally plausible given his character and the recent events.
I have enjoyed/appreciated all of Murphy's Law. Yes, the themes can lack appeal and some of the semi-regulars are of less interest than others over the five seasons, but at the end of the day, we watch Murphy's Law because of the wonderful Murphy character, which few if any could play as well as James Nesbitt.
If you have not seen Murphy's Law before, best to start with an earlier series, because there is greater diversity in a few one one-part stories than one three part story arc. The better theme music is there early on too. Once you have seen at least one earlier series, then these are then fine.