WCW Mayhem is a title you will enjoy more for the nostalgia than the actual gameplay.
As for the execution of the game itself there are some flaws any potential buyer should know beforehand. They look a non-realistic approach to the graphics. The background and characters look like something out of a cartoon and some of the wrestlers look generic while others are easily distinguishable to their respective look and persona. The sound is adequate for a Gameboy game of its time so I won't grip about it. The gameplay itself though is a straight up button masher through and through. You are able to punch and kick and when wrestlers get close they lock in a grapple which is just another button mashing competition. Occasionally weapons will randomly be thrown into the ring and you can use this to wear down your opponents stamina bar faster but there is no sound strategy. Also there is a lack of modes and music variety.
This game is a button masher with average production values and gameplay though I enjoyed seeing some of my old favorites featured as playable wrestlers. This game definitely isn't for everybody but hardcore WCW fans should kick a kick out of it nostalgia wise.