Having never read the le Carré source novel or seen the BBC miniseries from the 1970s, I didn't know the story or what to expect. I suppose I'm shocked neither by the critical acclaim it's been getting nor the negative reactions I've seen, especially from those who seem to have a different set of expectations for the movie. It's not a film for everyone, and that's alright. I wouldn't go so far as to use the word "convoluted," but it is undeniably dense, which at times works to its detriment.
I liked the film, overall. I found the performances uniformly stellar and the direction tight and, at times, alienating. I do not mean this as a criticism; rather, I think the style just reinforced how secret and secluded the lives of spies have to be. We only got small windows into the lives of these characters--and sometimes, no window at all--keeping the story focused on their work, by turns both dry and dangerous. It's a good way to keep a story focused; unfortunately, it keeps the characters from being well-developed. I spent much of the film reminding myself which character was which, which names went where, and why this was all significant.
I also felt that one of the prime suspects was underdeveloped, to the point where we knew it couldn't have been him simply because the impact would've provided no punch. I do not believe that everything needs to be spelled out for an audience; in fact, I prefer when it isn't. However, I do think there needs to be enough provided that we can not only tell characters apart but consider each a reasonable suspect, when it comes to mystery stories like this.
I feel like I've got more to say about this, but it's after one in the morning and it's escaping me just now. I may come back and add to this later.