MGS is not a niche game, and it is a system seller. Not to the level of say a final fantasy, Super Mario or Halo, but still a system seller.
What MGS is though (and what causes confusion I guess) is that it is as core as a core game gets. This is not a casual friendly game, unlike alot of core titles that can be played by the casual gamer. Heck I've seen a casual gamer pick up and play splinter cell, yet I was frustrated beyond belief when I played MGS1 for the first time (granted I was like 12).
I guess you could call it a niche game if you consider the core gamer market to be a niche market of gaming now (which thanks to the Wii that is not such a wild comment to make).
@Colonelstubbs: Careful there, you may create a postwhore with that last comment :)
