Phineas and Ferb is a fun, creative, intelligent, entertaining, well-done, and amazing experience for kids and adults, alike. The show is mostly great because it introduces high-concept sci-fi elements such as quantum mechanics and temporal paradoxes in a way that kids can understand. They do a good job at explaining and educating kids in a way that's not condescending and not boring. It's just fun.
Personally, the show was a staple of my childhood. I remember when the show was first coming out and I was obsessed with it. I watched every episode and had a bunch of toys about them. When I grew up and became an adult, I revisited my childhood favorite and found myself enjoying it once again for mostly the same reasons, although there were some new aspects of the show that I had come to appreciate as an adult.
The jokes are really witty and smart and they almost never fail to land. I can mention a lot of gags and zingers in the show, which would make even a full-grown adult laugh because they're just that intelligent. Every episode has a song, which is really catchy and everyone is different than the other. You'd think that with all the songs being written by the same people, they'd all start to sound the same but nope. The songs range from rock to pop to country to rap to techno to what-have-you and they're all total earworms.
The main characters (Phineas, Ferb, and their friends) rarely go through any conflicts or experience any negative emotions. In fact, they seem to have no character flaws whatsoever. Yes, they do in fact, qualify as Mary Sues (a complaint many marketers and show developers have expressed with these characters). However, the show still manages to be fun and exciting because of the world around them. It's not about the struggles of Phineas and Ferb. Rather, it's about the cool things they build. Conflict comes from the side plots, featuring Perry and Doofenshmirtz as well as Candace, who experiences tons of negative emotions and exhibits enough character flaws to make up for both of her brothers.
The show can get really formulaic as every episode has the same basic plot outline, but they have plenty of once-in-a-blue-moon subversions and twists on the plot, where the formula gets turned on its head for an episode making for an interesting watch. There's also many episodes that break away from the formula entirely to tell a completely different story and they're all really good. Personally, I can't think of a bad episode (except "Spa Day". Don't even bother watching that one. I mean, probably do watch it if you're binging the series, just for completion sake, but ... you know. You're not gunna like it.) Other than that, every episode is a sinch. ("Raging Bully" kind of sucked too. I mean, the jokes were funny and the side plot with Perry and Doof was on-point, as what Candace's story, but the main plot was absolutely horrible just ... HORRIBLE!!!)
All in all, despite its minor faults (that are few and far between by the way), Phineas and Ferb is a golden TV show that everyone should watch and I would highly recommend the show and all of it's affiliates to anyone I meet who's in the need of some riveting television.
YAY PHINEAS AND FERB!!!
P.S.: Also, if you enjoy Phineas and Ferb, you might also like it's sister series, Milo Murphy's Law. It takes place in the same universe and is just as awesome but that's a review for another day.