I had Mayhem because there was a promotion with Surge (ah, Surge, don't get me started) where WCW Mayhem was one of the prizes. I won 3 different copies (I drank a lot of Surge, apparently). The hardest person to unlock was Chris Jericho. To get him, you had to participate in each of the backstage areas. Which was time consuming because the backstage area you got was random each time. I got good at it though, unlocking every wrestler. I didn't know who most of them were, because I never really watched WCW. Joe was right, there were a lot of wrestlers. Unlockables were "Mean" Gene Okerlund, Arn Anderson, Bam Bam Bigelow, Barry Windham, Bobby Blaze, Bobby Eaton, Chris Jericho, Doug Dellinger, Eric Bischoff, Jimmy Hart, Lash LeRoux, Ric Flair, Rick Steiner, Scott Hall, Sgt. Buddy Lee Parker, Sonny Onoo, and Wolfpac Sting. And in the game from the start there were Alex Wright, Bobby Duncum Jr., Booker T, Bret Hart, Buff Bagwell (liked to use his music for entrances), Chavo Guerrero Jr., Chris Benoit, Chris Kanyon, Curt Hennig, Dean Malenko, Diamond Dallas Page, Disco Inferno, Eddy Guerrero, Ernest Miller, Goldberg, Hollywood Hogan, Horace Hogan, Juventud Guerrera, Kaz Hayashi, Kenny Kaos, Kevin Nash, Kidman, Konnan, La Parka, Lex Luger, Lizmark Jr., Norman Smiley, Prince Iaukea, Psychosis, Randy Savage, Raven, Rey Mysterio Jr., Saturn, Scott Norton, Steve McMichael, Stevie Ray, Sting, and Wrath. I liked to play as Psychosis. He seemed the easiest to win with. I never got the code for masked Rey Mysterio to work for some reason. I seem to remember it was supposed to be something like "MSKEDLTLRAY". None of my friends ever wanted to play it because it wasn't a WWF game, so the differences in controls between the two confused them. I was of the opinion that it actually meant I could beat them for once.
Haven't gotten online yet (which I've heard is actually pretty lacking in MK:DS, hopefully this is just a "first step" before Nintendo really gets an online network in place for future DS games/the Revolution)