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. "Mission Marvel" is one of them, with the combination of the 'Phineas and Ferb' regulars and Marvel superheroes. Not an original concept, but a genius one. 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.
"Mission Marvel: Parts 1 and 2" is a fantastic episode. Not quite one of my favourite episodes of 'Phineas and Ferb' overall, but one of the best, funniest and cleverest of the later episodes and certainly of Season 4 when the show was not quite as fresh as it was. It was a creative setting for the show, even if a familiar one in animation, and had a clever plot cleverly executed. There is a very 'Phineas and Ferb' feel, but with more than enough to set it apart.
Perhaps there are a couple of characters too many and some of the superheroes make bigger impressions than others.
However, so much works wonderfully. It 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.
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, the setting is very imaginatively and affectionately done and the references were witty and makes one nostalgic for when they were younger.
Loved the characters, can never get enough of Perry and Doofenschmirtz and of the superheroes Spider-Man is an immediate standout with some funny lines. The voice acting is spot on as is the character interaction.
Summing up, excellent. 9/10.