It has so many twists and turns, double and triple-crosses that you don't know whether you are coming or going half of the time. As a writer it was the plotting of this twisty-turny tale that immediately grabbed me. A star-studded cast (of mainly villains — James Coburn, George Kennedy (who makes what has to be one of the best entrances in ANY film, ever), Walther Matthau and the Mr. Sneezy-alike Ned Glass). Hitchcock's direction could never be termed 'slick' in my opinion, but Stanley Donnen's approach to the Hitchcock-ian thriller, definitely is that.
Again though, it's Grant's effortless characterisation of a man, who right until the end you don't know where he stands, paired with Audrey Hepburn's innocent and guileless Regina Lampert that lifts this film to stratospheric heights. They both get to show off their comedy chops, as well as deliver some complex characterisations as well as proving why they were two of the best romantic leads in hollywood at that time. I don't know how many times I've seen it now (must be twenty-plus!), but each and every time I get hooked in by the story, the brilliant characters and the excellence of the cast as a whole.
The final chase scene is as tense now as when I first saw it.
What's not to love?