Darkness at Noon is one of a class of novels, mostly prison and interrogation things, in which all is just so hopelessly restrictive and cramped - so lacking in even the smallest victories. 
 So, it's not fun. And it's not particularly new. 
 Koestler, though, was an early-adopter communist who had suffered through such imprisonment. He developed a main character, Rubashov, who swings from stoic to frantic and foundering to scheming wildly and naturally. He's human. Smarter than most and with stronger commitment to his beliefs than most, but human. 
 Better than that, Koestler is able to let these same glimmers of humanity shine through all of his characters. The interrogator slips from behind his rigid facade. The never-seen fellow prisoner in the next cell, there for a political offense that doesn't mesh well with Rubashov's, can't hide the frailty and warmth he'd prefer to deny even behind a brick wall. All battle to subvert their natural instincts in order to act according to their ideals and all fail a little and succeed a little. 
 Lastly, this obviously could have been a bit too talky, but it's got a good drive. Koestler keeps weeks long interrogation moving and provides a bit of a symbolic history lesson (if you were wanting to know a little bit about what it was like to be on the wrong side of a Stalinist purge. And I'm sure you were.)