Bosley Crowther raved about The Apartment in the New York Times back when it was released:
"You might not think a movie about a fellow who lends his rooms to the married executives of his office as a place for their secret love affairs would make a particularly funny or morally presentable show, especially when the young fellow uses the means to get advanced in his job.
But under the clever supervision of Billy Wilder, who helped to write the script, then produced and directed "The Apartment," which opened at the Astor and the Plaza yesterday, the idea is run into a gleeful, tender and even sentimental film. And it is kept on the side of taste and humor by the grand performance of Jack Lemmon in the principal role."
Roger Ebert agreed and named it one of his Great Movies noting in particular the timelessness of Billy Wilder's films:
"In observing that The Lost Weekend hasn't dated, I could be making a comment about Wilder's work in general. Even a lightweight romantic comedy like Sabrina (1954) holds up better than its 1990s remake, and the great Wilder pictures don't play as period pieces but look us straight in the eye. Some Like It Hot is still funny, Sunset Boulevard is still a masterful gothic character comedy, and The Apartment is still tougher and more poignant than the material might have permitted. The valuable element in Wilder is his adult sensibility; his characters can't take flight with formula plots, because they are weighted down with the trials and responsibilities of working for a living. In many movies, the characters hardly even seem to have jobs, but in The Apartment they have to be reminded that they have anything else."
Finally, here's the recent episode of the filmspotting podcast wherein they discuss The Apartment. This was part of their recent Billy Wilder marathon, which followed their Ernst Lubitsch marathon (which included a discussion of last week's Classic, The Shop Around the Corner) and a tie-in with the class on Wilder they taught this spring at the University of Chicago.