Disney has so few animation shows these days, and some of the "live" shows have characters that don't get along, over-act (poorly) or are just brats. In Phineas and Ferb the characters have character!! Even the bully and nerd get along as they act out their stereotypes with a sense of duty. Baljeet, "If I were dangling from a ledge he'd surely save me with a wedgie" ("Lotsa Latkes").
It is hilarious! The show has been a favorite for my son spanning his pre-K and elementary school years. I'll admit this is one cartoon that I don't mind watching with him.
Phineas and Ferb spend their summer vacation (which seems to never end, despite the theme song!) making wild contraptions, monster truck parks, teleporters to mars, an Edge-of-Insanity-Kiss-your-Butt-Goodbye-Gravity's-A-Stone-Cold-Sucker-Nightmare-Rail-Skate-Track-Obstacle-Course-of-Doom a more, while their sister tries unsuccessfully to bust them! And, Doofenshmirtz makes another hilarious "inator" after another.
Phineas and Ferb just makes me laugh and smile. The kids love them, the humor works on many levels, and it's safe fun entertainment for everybody. One of the few kids shows I don't mind watching, even in large batches (on a sick day or during the blizzard or the like.)
The music is brilliant too - catchy songs in all sorts of styles, and the family loves each other and is genuinely fun to spend time with. The shows where the parents get more time are some of my favorites even though they're never the main focus.