I came across "MI-5" (known as "Spooks" in the UK) by accident on Netflix a few years ago and have been watching every episode as soon as it comes to Netflix streaming ever since. I'm sad to hear that series 10 will be the show's last, but you can only do so many terrorists plots before they all seem the same.
It didn't take long to see that this was the most unpredictable show that I had ever seen. You never knew what was going to happen and who was going to live or die. All the old tell tale signs of plot did not apply. That made it a must see for me, and at times, kind of distrubing.
Many folks in the U.S. compare it to "The Wire". Here's a quick review of the show from The Boston Phoenix that hits on that topic:
Like the one American exception, The Wire, MI-5 works hard for believability; and like The Wire, it has so many characters with similar haircuts, skin color, and accents that at first it’s confusing. But by episode two, viewers have figured out that the “main” character is Tom Quinn, whose girlfriend calls him Matthew, thinks he’s an IT geek, and doesn’t know he’s a spy. Tom is played by Matthew Macfadyen — Darcy in the Keira Knightley Pride & Prejudice and Arthur Clennam in WGBH’s current Little Dorrit. The rest of the cast, whose number includes a multitude of non-regulars, are universally solid performers, among them Peter Firth (as the boss) and Jenny Agutter
—-
US viewers will have to get past the deliberately anxious pace, as well as both the clipped British accents and the horribly mimicked American ones, but once you buy into it — period-piece production tricks and all — MI-5 is one thing television almost never is: genuinely suspenseful, engaging, and exciting. Call it manipulation if you like, but with the right material, it works.
Here's the trailer for the first series:
Just a warning for Americans: This series is not very black and white as far as good guys / bad guys, and the Americans are not always portrayed in the most positive light.