I've read this a few times when I was younger, but never really enjoyed it. I decided to give it another go.
It is a step-change from le Carré's previous two novels, but maintains a plot link to Call for the Dead. It seems a bit ironic to complain about the darkness of a le Carré, but there you go. Perhaps in part it's because I think of le Carré as painting shades of grey, but here (and in Call for the Dead also, with hindsight), East Germans are portrayed in a distinct and unambiguously unpleasant dark hue. Of course, this is a novel of time and place, and le Carré certainly had experience of working in Germany and Berlin at that time. Another reason for my perception may be the peripheral appearance of Smiley, whose motivation is left characteristically ambiguous, though in all probability it is in keeping with the twist and twist again of the plot. I'm not a fan of the ending, either: it's an understatement to say it doesn't tie up all the threads, if the reader is to accept everything told up until this point.
On this reading, perhaps because I had low expectations, I found it better than I remembered, but it's certainly not my favourite le Carré (there again, Tinker, Tailor is hard to beat), and I do find it odd that this really launched his writing career.