First of all, I actually enjoyed this mini-series, which, as has been noted, is elegantly produced and, on the whole, well acted. The costumes and settings are also excellent and evocative of the period. Michael Keaton is especially good in the role of chain-smoking James J. Angleton. And Tom Hollander, who seems to be making a career playing various Cambridge spies (He still has to play Maclean, Cairncross, and Blunt [which will require a real suspension of disbelief].), is brilliant (as usual) in the role of Philby (whom the writers have coyly called by one of his middle names, Adrian, so that viewers who may be only vaguely familiar with the early history of CIA will not guess he is Kim, the British Soviet Mole). Hollander plays the spy with understated charm, and his suggestion of Philby's stammer never slips into parody.
Good points being acknowledged, I now come to various aspects that have been already stated in other reviews: the overproduced music (which sometimes drowns out crucial dialogue); the handsome but rather wooden hero, who ages twenty years only in the steel color of his hair; and the confusing flashbacks, which cloud the narrative. As one who is fairly familiar with the historical background, I was also annoyed by details, which I admit are picky: if the heroes graduated in the Yale class of 1954 (as has been indicated), Philby, along with Burgess, had vacated Washington in 1951; by 1954, Burgess was in Moscow and Philby was being interrogated in London, cleared, and rehired by SIS and sent to Beirut under cover as a foreign correspondant, so there is no way that the young Yalies would have been in on the Philby debacle (One of them is depicted as delivering whisky and other goodies to him in Washington.). I think I prefer the "Good Shepherd's" solution of giving real people false names [We still knew that Matt Damon was Angleton!]. Then, none of these plot points would make the slightest bit of difference.
Again, I enjoyed the mini-series, which, under the aegis of Ridley Scott, sticks to standards that are much higher than the usual U.S. television fare. I did not think the series was as bad as some reviewers have judged it, but then, I think it could have been far better with a little more planning and a lot more editing.