Yes and no.
I don't think the Fantastic Four itself is worth the cost. They were always held a little bit separate from the Avengers and the X-men continuities. Sure, when things got really heated, they'd pull in Reed Richards as a kind of deus ex machina super-genius but the team itself isn't vital.
They're also one of the harder properties to handle well, being somewhat centered around that deus ex machina of technology I mentioned. Most of their battles were won by the rest of the team buying time for Reed to invent a new kind of machine that would solve all their problems instantly. That's not especially compelling or dynamic. The family conflict-then-reconciliation story-line is another cliche the series leaned on heavily but it's been done to death in cinema. The property has also been devalued a bit by Fox, which is a shame.
The "yes" answer, though, comes from the associated villains, especially Doctor Doom.
Doom is one of the best characters in comic book history. He definitely is compelling, if portrayed correctly. Marvel needs him back. In the hands of someone who understands him, who can bring out the menace and strength of will he possesses, Doctor Doom could be one of the more important characters in the Marvel cinema hierarchy. He sure as hell is more than a side character for the Fantastic Four.
