Being niche is linked to sales, it's not defined by it. It's almost a tautology that if something is limited in it's appeal, it will be limited in sales. The degree to which that happens is obviously different, but honestly I think your own examples actually prove my point. Yeah, Disgaea, Katamari Damacy and Castlevania, they're really mainstream...
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.