I never actually owned a Sega Genesis growing up, my family's mostly been about Nintendo. I remember a time when I was probably no older than 7 or 8, I went and visited my cousin who owned this game. I was blown away by how freaking sweet this game was.
As soon as I got home I begged my parents to take me to blockbuster (hah, blockbuster) so we could rent the game for my Nintendo. What I returned home with was not the same game, it was some incredibly crappy, half aborted fetus of an X-Men game. It was that day that I realized the difference between 8 and 16 bit gaming.
About 5 years ago, I was at a Gamestop and saw this game sitting on their rack of old school games, I had to have it. I asked the guy there if they had a system available for purchase...I was in luck! I walked out with a Sega Genesis, two controllers, and some games (including my beloved X-Men) for right around $17.
Why This Game Was/Is Sweet
First off, you get to choose from all of your favorite X-Men good guys (how good they are is arguable by what series of the comic you refer to, but I'm not getting into that): Wolverine, Nightcrawler, Gambit, and Cyclops. Each X-Men had the use of their powers of course, but this wasn't just some little laser blast or regular teleportation power. You had advance options such as holding down the fire button for a more powerful blast or the ability to transport a friend with you (if you were playing 2 player mode). Sure using the more power attacks used up your mutant abilities faster, but they regenerated eventually, and you felt like a total bad ass one-shotting some of the tougher bad guys so all was good!
You also had the ability to switch between all four of the X-Men (or two if you were playing co op), plus there were a couple of the supporting characters that you could use once a level, they'd come in and beat down whatever was on the screen.
The second reason why this game is sweet is that it was one of the first games (at least that I can remember) that broke the fourth wall. For those of you who don't know, the fourth wall refers to the imaginable barrier between an audience and it's target. For example, if you go and see a play, there's a fourth wall between the audience and the play being performed. The actors don't reference the fact that they're being watched, or ask the audience to participate in what's going on, doing so would break the fourth wall.
This game breaks the fourth wall once you're getting close to the end. Professor X tells the X-Men that in order to fix the issues going on with the danger room, the master computer must be reset after beating a simulation. So you play one more level, once you beat it, you need to gently reset your Sega Genesis in order to gain access to the final level where you can defeat Magneto and win the game! If you didn't get the subtle hint, you're doomed to play the same level over and over again.
C'mon, how freaking cool is that?!
Sorry this post is a little longer than most, I became enthralled with the X-Men's awesomeness!