It's a good read. Well written, but it's depressing (as is often the case in character studies). The spy service is demoralised (given the UK's lack of confidence over their position in the world, coupled with the knowledge that someone's obviously leaking given all the failures), and drifting (with no one really in charge -Smiley's boss was dying and his replacement -who eased Smiley out- plays favourites), but out of control (certain pet projects make all the running). To make matter worse, few of the employees are able to talk to each other in any reasonable way.
That having been said, the "investigative" elements (where Smiley investigates the spy) were good fun to read, even if it was pretty obvious who the spy was as the story progressed.