I don't know...despite the bad reviews, this is still one of the most satisfying 2-D action games for the NES that I have played. Great graphics (very large detailed sprites), dramatic 8-bit soundtrack, primitive RPG elements (an experience system that's tied to your score), a long campaign that traverses the entire solar system, and tons upon tons of boss fights. The difficulty level is just right too...the first few levels are laughably simple and repetitive, but slowly things ramp up until the final world (Planet X) where the aliens throw everything they have at you and an incredibly challenging King Ghidora awaits.  Anything bad?  Well...I do wish you could play as more monsters than just Godzilla or Mothra.  Also I would be lying if I didn't admit that the level designs were redundant.  But like great action titles like HALO, there's so much activity going on onscreen that you'll hardly have time to be bored.  I played it like mad when I was a kid and even nowadays, I still find it quite entertaining and challenging.