It sounds fun, but is it a crossover with Ultimate Spider-Man (the show) or just Marvel in general? I haven't watched Phineas and Ferb in ages - and even then, I've only seen like 3 or 4 episodes, but I might have to check this out.
Marvel comics in general. Did you look at the trailer?
As for my opinion, I'm on the fence about this but I think it's due to the more mature adaptions and characters of the Marvel franchise that are truly affecting this decision. From an a view as a fan of Phineas and Ferb, I think this will do great. From the view as a fan of Marvel comics, I think it will flop. Also, since I'm a Marvel fan-boy before a Phineas and Ferb fan-boy, I have a stronger feeling that this won't go over well.
As for why not a different Disney cartoon as opposed to Phineas and Ferb? Well, let's take a look at our other 3 candidates.
has a lot of potential for a crossover with Marvel comics due to it's eerie and grotesque tone however, the show is fairly new and thus there isn't enough reception to even consider a special yet. Fish Hooks
is an awful show to begin with and the only hero out of Marvel lore that would fit the environment is Namor the Sub-Mariner who I have a feeling that very few of the kids know. Kick Buttowski: Suburban Daredevil
could blend very well with Iron Man, Deadpool (awful example), Spider-Man, or the like. The problem? We get a decent and acceptable crossover but it's just more of the same of what the show still delivers. We want something fresh to be brought to the table and I don't see how Clarence using superpowers or a super-suit to preform stunts would change the show enough.
Also, I question, why is Hulk still the Hulk in the promotional picture if he had his powers taken away? Similar can be said about Iron Man. Oh well, we'll see what happens but I hope they fix or correct these errors.