The AI isn't very aggressive, so you don't have to go crazy pushing buttons just to land a few hits. Most of the enemies aren't very difficult, as long as you play pretty defensively. But this won't yield a very high score, and the final score determines which monster you fight last (out of eight, I think). Trying to unlock all of the beasties provides a little extra replay value.
The music ain't bad, but it's nothing to drool over either. The music may not be memorable, but I do recall that it has nice samples of the different creatures growls, screeches, and whatnot. I don't know if they are accurate when compared to the movies, but they fit nicely.
Has anybody determined the point behind being able to unleash Godzilla's roar? I could pull it off regularly in a versus game, but almost never when playing the computer. I always hoped that it would uncover some hidden level or something, but I never found anything.