There are quite a large number of 'Phineas and Ferb' that had great story ideas, managing to follow the basic formula while doing more than enough to give it variety. "Phineas and Ferb's Quantum Boogaloo" is one of them. It's actually one of the more unique and creative ones. Loved it when it first aired during an up and down period for me, and have re-watched it a number of times since as that's how positive an impression it made on me.
"Phineas and Ferb's Quantum Boogaloo" is one of those 'Phineas and Ferb' episodes that got better and better with each viewing and today it is still an outstanding episode. Have found out in recent years that it's a fan favourite, with many fans putting it in its top 10 and even Dan Povemire considers it one of his favourites. It is more than easy to see why and it is richly deserving of this acclaim, count me in as another person to consider it one of the show's best.
As to be expected from a 'Phineas and Ferb' episode, "Phineas and Ferb's Quantum Boogaloo" is excellently animated. Vibrantly coloured and the backgrounds has lovely attention to detail, plus it is visually quite ambitious for 'Phineas and Ferb' at this point of the show's run. The music is dynamic with the action and the theme song is insanely catchy with very clever and quotable lyrics that anybody still in school fishing for how to spend their vacation will relate to. One of my favourite things also about the episode is Doofenschmirtz's song "It's Been a Charmed Life", it is dripping with irony and in a way that is so delicious and hilarious.
It's exceptionally well written as well. The target audience is clear and there is something for kids and adults alike. The writing is smart and mature, while not being too complicated for younger audiences. While with enough complexity emotionally to not be simplistic for older viewers. "Phineas and Ferb's Quantum Boogaloo" is very funny (i.e. Stacy becoming president of Uruguay), Doofenschmirtz for instance is a riot, but also has a lot of heart. The idea was a good one and the story is imaginatively done and while a lot goes on it doesn't get over-complicated.
Character writing and the voice acting are spot on all round.
In conclusion, fantastic. 10/10.