[The Knack is] crazy, but not twee, since the whimsy has a good sound emotional springboard, in all the embarrassments and yearnings that beset teenagers in their efforts to reach one another. It's a very frank film, and it translates into modern terms the sexual grotesquerie that was part of the charm of the dear old Crazy Gang—as when the hero, clambering over some park railings to save the heroine from a fate worse than death, gets his fly-buttons hooked on the spikes, and hangs there immobilised. The film has a demented ubiquity of sexual innuendo: Tolen, offering to play Nancy some records, promises, 'You'll like Theolonius—he's deep—he's satisfying—', and Tom worries Colin by commenting, on the subject of elephant's trunks, that 'any limb that isn't in constant use atrophies and drops off'….
The erotomaniac humour is nicely counterpointed by a volley of anarchist-spirited jokes, with a double-bedstead weaving in and out of the traffic like a Kon-Tiki raft, and a real little classic gag about a kindly old gentleman gallantly helping pregnant [Nancy] across the road….
The weak point of The Knack is, precisely, a tendency to lose a scene's point in a fandango of gags. At times, certainly, it captures [the atmosphere of Ann Jellicoe's play, on which the film is based,] very well…. But the film's so funny that, as a whole, it dissipates the play's emotional voltage, its anxious strain and violence, all the eerier for being located in the style.
Raymond Durgnat, "'The Knack'" (© copyright by Raymond Durgnat 1965; reprinted with permission), in Films and Filming, Vol. 11, No. 10, July, 1965, p. 25.