Spooks is a fantastic series, and season 2 was simply awesome. There are some amazingly tense episodes, such as the VX gas episode, and the finale leaves on the edge of your seat, waiting for for season 3. The characters are developed from the first season as you see how the spy game takes its toll on their personal lives.
Spooks is kind of similar to 24, but runs at a slower pace without losing the tension. The script is measured and deliberate, with a limited amount of jargon. The focus is on the story and characters, with gadgets used sparingly and never dominating this superb series.