Verdict:

This show suffers from the same problem as many shows do: the directors and writers differ from episode to episode, which results in an inconsistent quality of the show.
Things that are not so great: mostly the first season where the action, the flow, a lot of the dialogue and some of the acting are below par.
In the first episode, there are three dudes shooting automatic weapons at a helicopter that’s slowly lifting off only a few meters away from them and has its side doors open, fully exposing the passengers. But the show had already declared the heli to be safe ground and so it gets away. A similar thing happens more than once later with a car. At one point a knife is thrown to kill a soldier (in case anyone still thinks that happens in real life). That stuff happens in campy Van Damme movies, but this show pretends to be more grounded.
I have not read Chris Ryan’s book on which this show was based but I do not believe that he, a real former special forces operative, would write something like “and then he threw a knife at the soldier”. It is like I am watching an 80s action movie, without the charm of Arnold Schwarzenegger.
The action of the early episodes lacks impact and feels unrealistic. Soldiers move slowly and the camera hangs still for a while, concentrating too much on their faces. This isn’t a peaceful conversation you’re filming, so give us some interesting camera angles, man. I cannot believe I am saying this, but this is the first time I thought the camera could have used a little more shake.
There is a general problem with the flow of the scenes. Again, this is mostly the case when something is happening and largely confined to the 1st season. There is a slight delay in actors’ reactions, where one camera shot will show something happen and then the next shot will show an actor react to what happened but there will be is a slight yet noticeable delay in between.
The dialogue is poor. Among other issues, the show has a bad habit of having its characters verbally clarify what’s going on, when it is not necessary.
The writing as a whole has its dips. Someone just located a place on the map and immediately proclaims that the ETA to fly to that location is four minutes. How do you know? Why not five minutes? One guy says to the other “It could be worse” and then they see an enemy helicopter appear. That’s a fairly cliché mild joke, but it’s fine if you stop right there. Instead, the other guy says “It just got worse”, which kills the joke. This show features things like giant red countdown timers on big screens in order to indicate a deadline. This is the sort of thing you would expect from a cheap show on the Sci-fi channel.
The quality of the acting varies, but during the first season it takes a dip. Many reactions from the actors did not feel natural, especially from Andrew Lincoln, who plays one of the main characters until the 2nd season, and who keeps raising his eyebrows and accentuating words so that all his dialogue might as well be “I am acting now.” He sounds like he should be recording an instructional audio tape.
Things that are pretty good: later episodes, most of the acting, the general style of the show, the ambiance, the humanity and some good writing.
The British have a certain sensibility and intelligence that displays itself when they write stories or shoot film, and this show is not an exception. Despite the occasional bad line, there is a sense of realism to the emotions of the actors, a sense of style, a sense of substance. There is a human side to everything that’s going on, without the need for the actors to overact and curse at the camera, which is the way an American show would probably handle emotion. The show knows when and how to be subtle.
There is also a certain rawness that I like. There is prison nudity, severed limbs and concealment of objects in…private places.
While the story can be dumb in the details, it is actually quite good as a whole. There were several twists that I did not see coming.
The aforementioned flaws of the show start to fade as the series progress and new directors and writers take over, though the problems do not disappear entirely.
If you are not sure about the show, I would suggest skipping the first season of it all together or at least starting with episode 5.