From time to time I like to rewatch these series. They just improve with age. It's not the overall plot lines that draw you in, it's the minutiae of the production and of course the performance of Alec Guinness.
Wikipedia has these comments: In a retrospective review in The New York Times, Mike Hale lauded Guinness's performance, ("It's conventional wisdom that Guinness's performance is a landmark in TV history, and you won't get an argument here, though if you're watching it for the first time, you may wonder at the start what all the fuss is about.") and cited the production's pacing versus current techniques, stating, "Audiences used to the pace of the modern TV crime or espionage drama will need to reorientate themselves."[15] Retrospective reviewers favourably compared the series with the 2011 film version, also citing le Carré's praise of the original and referring to Guinness's performance.[16][15][17][18]Le Carré cited the series as his favourite filmed adaptation of his work, attributing this to his experience collaborating with Guinness "
