I was honestly looking forward to this movie. I loved the cartoon for being a breath of comedic fresh air in a medium that seems obsessed with trying to pass off weirdness AS humor. Phineas and Ferb is a great series, the characters are all strong, the jokes range from usually very good quality to pee-your-pants hilarious, and the songs are a welcome bonus as many are so insanely catchy.
The movie drops none of these elements, takes all the good elements of the cartoon and plays them as best as it can.
Granted, it's not genius per say, but it is extremely good. Better than most TV Movies that I can think of.
One of the reasons I like this film is that I really like the idea of alternate universes because it can open up so many ways to play with your characters in alternate interpretations. While that isn't done much here, it's done enough that I enjoy it. The joke about the alternate Doofenshmirtz's backstory is one of the biggest laughs in the movie I think.
This movie also broke one standard of the cartoon, that the boys never find out about Perry's agent status. While the ending is what you would expect, it's done well and I like it.
The Songs are also good. I find that all the songs are enjoyable, though some are more memorable than others (as to be expected.) "Robot Riot" and "Everything's Better With Perry" are the two I find myself thinking of the most.
The cast is as enjoyable as ever. Phineas and Ferb are their typical kid genius selves and Perry is his typical awesome self. They react naturally to the events around them, though Phineas is given more of a emotional reaction due to Ferb being, well, Ferb.
This movie is a good adventure that contains most of the good stuff that makes Phineas and Ferb the great series that it is.
All in all, I give an 9/10