John Hannah shines in anything he does. Amnesia had us on the edge for 3 episodes, since it showed over a 3-week period here on TVO! Patrick Malahide was also superb as his boss. What these guys can do with their facial expressions! (applies to rest of cast also).
General comment: British suspense-mysteries are compelling because the characters are so complicated. In fact, the cops are sometimes more messed-up than the guys they're trying to catch. The plot or the 'whodunit answer' is often secondary. If you try watching typical American CSI-type shows afterwards, they almost seem laughable.
What gives Amnesia such a punch is that it does have an intriguing plot and outcome also. The movie ends, but the story doesn't. Hope you enjoy as much as we did.
Added note: a few are questioning the conclusion or ultimate solution to the mystery. The Brits don't worry about whether the parameters of their plots are rational-they just want to surprise you! Stop worrying!