Good, but not without flaws.
First off I noticed a loss of characters. Johnny Cage, Raiden, Kung Lao and Baraka were missing. I was really surprised that they took out Johnny Cage and Raiden, because these starred in MK1 and were my favourite characters. I thought if they needed to take out characters, they should've taken out one or both of the sisters, Kitana and Mileena, because they're both pretty much the same. They possibly should have also taken out Reptile.
The graphics and the animation was surprisingly good for the Game Boy, but the flaw is that all the ninjas are really hard to tell apart while fighting. So are the sisters. But the thing that saved that was the inclusion of "Toasty!"
The sound is also pretty good, but they spent more time on music, rather than sound effects. The characters don't go "Ow", "Oo", "Argh" or "Ohhhhh".
But still it is fair quality.
The controls actually work quite well, although there are only two buttons for punch and kick, but there are none for block, but in a way thats good, because in the arcades I found it unbelievably annoying when the computer blocked every single move. Also the few buttons make it a lot easier figuring out moves.
All in all, if you are fan of Mortal Kombat or of fighting games in general, I highly reccomend getting this, because the only real flaw is the loss of characters. But if you are not, steer clear.