I really like K.C. Munchkin and I think it's the best of the "Pac-Man clones" if you can call it that. I agree with what RickR said that it's "a nice twist on Pac-Man" because there's enough different about it to make it a different game, even though its still a maze game with a ball gobbling dots.
Two memories stand out to me about K.C. Munchkin:
1. I remember thinking K.C. Munchkin was the reason I wanted to get an Odyssey. It's the only reason I can remember. Guess that qualifies as a killer app.
2. I remember playing K.C. Munchkin at J.C. Pennys and thinking the next version of Pac-Man would have to use some of these ideas of a changing maze, pellets that open and close things and so on.
I've never played the 7800 version of K.C. Munchkin but PacManPlus does awesome work with everything. I can't think of anything he's made that's been a disappointment. K.C. Munchkin is on my list to pick up, It looks like a faithful port with some nice improvements and looks better than the Odyssey version in all the right ways. My gripe isn't with the game at all, it's that I wish more were being done with 7800 that could best the NES rather than the Odyssey. NES-quality games are needed on the 7800, but that's a different topic all together.