I wouldn't feel too bad if you find it a little hard to piece together this film while you're watching. It's a complex picture, taken from source material by big water-muddier John le Carré. A meeting gone awry in Budapest is far from what it seems originally, where it's hard to tell who's connected to whom and who's setting up whom (if at all) and what the significance of this event is. (All does come to be explained, maybe even a little too thoroughly.) The experience of watching the film involves both keeping up with what's currently happening on screen and making sure you understand that's come before. This may be irritating to some; some proponents of the movie have neglected to understand this.
The film follows the efforts of dismissed operative George Smiley (Gary Oldman, solid but 90% flavorless) and his still-in-the-system assistant Peter Guillam (Cumberbatch) to track down a double agent high up in "the Circus" (i.e. MI6). I found their antics (and Smiley's day-to-day life) relatively uninteresting, and was much more enthralled by the people they come into contact with. The most prominent one is Ricki Tarr (an engaging Tom Hardy), who, on a dead-end assignment, ended up finding some extremely valuable information via his observation target's abused wife Irina (Svetlana Khodchenkova), who becomes his love interest. Also involving is Jim Prideaux (Mark Strong), mostly because it's surprising what happens with him.
Though what transpires eventually is affecting and thought-provoking, and though the film feels like it has much to it, much of it is tedious and muddled (in plot and in assembly), with one Christmas party scene used as a flashback maybe a few more times than necessary. The last song in the film threatens to drench the film in nauseating Style as well. I can understand what people appreciate about "Tinker Tailor Soldier Spy" but I have a hard time loving the film to the degree that some do. B-