Sin City made use of b&w quite well, capturing the feel of the comic book perfectly.
And look at a film like The Seven Samurai that's shot SO far ahead of what should have been technically possible at the time, and you can sometimes forget the film is in b&w at all.
Pi was a great film, I think being shot in color would have detracted from the vibe of the movie.
I JUST remembered American History X and Schindler's List were b&w as well, they were done so masterfully I had to see them on a list to recall they were not in full color.
Natural Born Killers, Clerks, Man Bites Dog - now they are really flowing. Color means nothing if the content is good enough.