This is indeed classic Hitchcock - a gripping and characteristically well-executed tale composed of just the right mix of humor, suspense, and irony. I hardly feel justified in faulting this film on any point, though one might conceivably point out that it does take a lengthy period for it to really get started, with the inevitable effect that it does tend to drag a bit at times. It might also be argued, however, that this is simply part of Hitchcock's genius - allowing the viewer to get comfortable before immersing them in the action.
Review of The Man Who Knew Too Much
