Really the only bad thing you can say about "I Spy"
is that it spawned the awful Owen Wilson/Eddie Murphy movie. The series itself is a classic and stands head and shoulders above most of its contemporaries.
Robert Culp isn't a typical TV ladies man; he's a little too tall and angular. But he's a convincing man of action and has a great wry delivery. Pairing him with the rising star Bill Cosby was genius. When "I Spy" premiered, Cosby was in the middle of his streak of Grammy-winning comedy albums - it must have been quite a surprise to see him exercise his dramatic chops on TV. Cosby is not used for ridiculous comic relief in the way that most comedians in action movies are today. He and Culp are equals - Kelly Robinson and Alexander Scott are both highly intelligent, capable agents and it's nice to see that racial dynamics in the US didn't force Cosby into a "sidekick" role.
The scripts are layered and complicated - they tend to be far more interesting than a simple "catch the bad guys" routine. Often, "I Spy" forces Kelly and Scotty into tough moral spots. For them, the spy business isn't glamorous; it's rough, violent and frequently depressing. Some of the best scripts are written by Culp himself - one of the first instances of a star writing for his own series.
Because of the down-to-earth quality of most of the scripts, "I Spy" isn't as much fun to watch as, say "Mission: Impossible" or some of the other 60s era espionage shows, but it's a better overall series.