There's nothing particularly noteworthy about X-Men: The Official Game for the Game Boy Advance. As is typical of movie tie-ins, it's a by-the-book side-scroller with all of the requisite combat and exploration that you'd expect from that sort of thing.
Combat mainly involves beating up hundreds of cloned goons.
The main mode consists of 25 side-scrolling levels. Some are mazelike and filled with high-tech enemies. Others are large rooms occupied by one of Magneto's top lieutenants, such as Sabertooth, Juggernaut, Pyro, or Multiple Man. If you find the secret areas in the story mode, you can unlock two bonus modes, called arrangement and danger room. These modes let you play individual levels and adjust how many enemies you fight when doing so.
For the sake of this game, the four good characters are Wolverine, Colossus, Iceman, and Nightcrawler. You control one character at a time, but you can instantly swap to a different character just by pressing the left shoulder button. Each character has a unique ability that's useful in certain situations. Wolverine can heal automatically, Colossus can punch through weakened walls and floors, Iceman can attack enemies from a distance, and Nightcrawler can teleport through reinforced walls.
Like in any side-scroller, the general idea is to run, jump, and punch your way through the standard levels and defeat each boss that's waiting in the roomlike levels. Beating up on cloned fodder enemies does feel tiresome at times, but the boss encounters make up for it. Their behavior and superpowers are right in line with what has been portrayed in the comics and movies.