X-Men was the first, and as far as I know the only, six-player arcade beat-em-up. It accomplished this feat via a giant cabinet with dual monitors across which the playfield stretched.
This one lets you play as Cyclops, Storm, Wolverine, Dazzler, Colossus and Nightcrawler as you bust up the forces of Magneto, who is (per usual) out to take over the world. Aside from the extra two players and the novel split-screen view, this game is no different than any other beat-em-up from the period. You bust through an army of cloned enemies, at the end of each level you face a boss (who progressively flashes more and more as they are damaged), mashing both buttons together performs a super move at the cost of a chunk of your life, and so on.
The issue with this one is that it apparently thought it could skate by on novelty alone. The art in the cutscenes is very nice, but aside from that and the six-player gimmick the game is mediocre, and actually a cut below Konami's other brawlers such as Ninja Turtles and The Simpsons. The enemies consist of a rote overabundance of Sentinels, there's hardly anything going on in the backgrounds in most areas and the music is a somewhat irritating dance style not up to Konami's usual standards. Apparently graphics and effects had to be sacrificed in order to make the dual-screen action work, as aside from the rather uninspiring backgrounds the character sprites are also rather small and low in detail.
Furthermore, the X-Men are ridiculously weak in this one, frequently getting taken to school by the average Sentinel. Each player has only a three-punch combo to attack with, making them all feel remarkably the same. And there is no use of their powers outside of the two-button special, which makes them perform only one simple attack that clears out a few enemies immediately in front of you. To have an X-Men game where the powers are barely used and simple punching and kicking is what you spend 90% of the time doing (as in any other generic beat-em-up) seems remarkably short-sighted, perhaps even lazy. There are no Street Fighter-style moves either, no dashing or anything extra of the sort. Just punch, punch, kick and the occasional super move.
Unfortunately, the game thus ends up being the very definition of mediocre, repetitive button-mashing. It's really a shame, and one of Konami's few failures with a licensed property in the arcades (though the game was fairly commercially successful and still has its fans).
* Gameplay Video