A comedy about something as important as the safety of the "free world" has some hurdles to overcome coming into it. Get Smart succeeded in its broad mix of approaches to comedy, including physical pratfalls, satire of world affairs and movies and other television of the time, improbable situations, and pretty strong character development for a comedy. Surprisingly consistent through most of its run, probably its strongest aspects are the Smart character (someone who can be suave one moment, and incredibly dense the next), his relationship with the Chief, and the very talented Barbara Feldon as 99. Unfortunately in the later years, after the marriage of Max and 99, some things were lost, and by the final season, the character of Larabee was elevated in prominance far too much and had taken on a sense of stupidity that even surpassed that carefully balanced for the Smart character. Some formula catch-phrases used by all characters had also grown a little tired. The most memorable lines remain those uttered by personalities like Captain Groman in "Ship of Spies", by Harry Hoo, or by the "Craw".
People often talk about the Get Smart's influence on the spy genre, but I also think it was a precursor to a style of comedy made famous by Police Squad, Airplane!, etc.