It assumes that its audience have some sense of Cold War history. In fact it seems targeted at baby boomers - those who read the book and saw the TV series but have difficulty remembering the intricacies of the plot and characters. This reviewer was in that category like many of the cinema-goers attending.
Gary Oldman as George Smiley does a serviceable but lacklustre job. Alec Guinness showed in the 1979 TV series that dour doesn't have to be bland. At times it seemed that Gary was mimicking Guinness. The rest of the cast are real pros, though John Hurt seemed too debauched as Control, but doesn't he always. At the other end, Colin Firth lacked a rough edge in his role.
Have to wonder what the current generation would make of this nostalgia flick, if they bother to see it. Director Tomas Alfredson has created a world that never existed yet somehow rings true.
I was reminded of Sir Humphrey Appleby's great line in Yes Prime Minister when he learns that his predecessor was a Russian spy. How could "one of us" be "one of them"? Unfortunately the film leaves that question unanswered.