It was a great synchronicity that this popped up on one of my reading lists right now, as one of my gaming groups is about to embark on a game of Cold City, set in post-War Berlin, playing representatives of different countries in BPRD-like surroundings.
But my spycraft is sadly lacking, so reading Tinker, Tailor, Soldier, Spy was a nice way to get a tiny bit of the taste, although this book takes place later in time, and only in England. I enjoyed it both for the story, and for the atmosphere.
I did have some difficulties with the writing at times, though. I found it difficult to tell sometimes whether something was a flashback, a story recounted, or in the present. I often had to stop and go back a few pages to try to figure it out, and still had difficulty. If this was le Carre deliberately trying to blend the past and the present to make the point that they are inextricable, good job. If it was just a sign of sloppy writing, not so good.
Also, if you get this in a later edition, don't read the foreword le Carre added. If you're paying attention, he pretty much gives away who the mole is.
But this novel does a good job of showing a treacherous world, in which anyone you speak to could be a double agent, and is fascinating in how it explores how you would track a mole without letting him know that you're doing so. (Particularly given that you're not sure who he is.) The claustrophobia and constant tensions are well elucidated.