It was pretty damn unnerving when I first ran into him/it. While the intermixing voices isn't a
new concept, it was great and not too cliched back when I first played it.
As for the "best boss ever", it's hard to say but I'm thinking back through the games I've played and he's probably one of the more original ones. Obviously as far as boss battles go, it's been outclassed many times over by newer games, but the actual CHARACTER is pretty rad.