Sure, it's pat and simplistic in places and the plot's a little daffy, but it has three major things going for it: an amazing dog named Friday, a delightful performance from veteran Edward Arnold and fine direction by Fred Zinnemann. It could've easily been filler, but Zinnemann has too much respect for his craft and the material to allow that to happen.
As others have pointed out, that dog really is something and nearly steals the show but Arnold is every bit as good. He is particularly amusing in his role within a role where he pretends to be an eccentric, ill-tempered uncle in order to foil the bad guys' dastardly scheme. (And that scheme is a big time McGuffin, no more than an obviously slight excuse to get all the conflicting characters under one roof.) Arnold's Cat & Mouse games with main villains Katherine Emery (resembling Mercedes McCambridge both in looks and delivery) and over-educated "butler" Stanley Ridges are tense and clever.
Zinnemann really shines in one ingenious scene set in a pitch dark basement. Arnold, playing a super smart blind sleuth growls "In the dark! In my kingdom now!" and proceeds to outwit a trigger happy thug. Not unlike the Coen brothers' "Blood Simple" 45 years later, the only light is provided by a number of randomly fired gunshots. Not surprisingly, this technique is effectively taut and unnerving. If you weren't aware who the director was at that point, it's the sort of thing that makes you go running to your film guide thinking "Whoa. Who directed this?"
