I got into this show about three years ago, and it's simply awesome. The characters are enjoyable and aren't mean spirited, they're actually a lot of fun to watch. The show may have a basic formula, but there needs to be credit with the fact on how successful the formula is.
Another thing that this show surprised me with is how good the songs are. I generally hate songs in shows, but the songs in Phineas and Ferb are really good and some of them are hilarious and catchy. Phineas and Ferb has done just about all the types of songs you can think of, and that's what makes this show so special.
Also, many cartoons have been suffering with seasonal rot, but this show is on of those instances where the series is just as good now as the first season. There never was a bad season, all three of them so far has been spectacular. I hope this show continues on being successful, it truly deserves it. I never cared to go to Disney Channel until this show came on. And it's the only cartoon besides South Park that I tune in regularly to see.