Marvel’s durable sci-fi comic about a league of teenage mutants with wild superpowers yields another warm-weather action flick, though the light tone and modest character development of X-Men have given way to an avalanche of digital effects and a bloated symphonic score. The screenplay, by a five-man committee that included director Bryan Singer, gives more time to the mutant community’s various powers. The demonstrations of the latter are well rendered and often quite funny, but they push this into the realm of the Saturday-matinee serial: whenever peril looms, some misfit kid’s freak abilities are just the ticket to dispel it. As in the first movie, Ian McKellen and Patrick Stewart are trotted out periodically to add a little gravitas. 124 min.