I thought b&w only was a gimmick until I saw the results. Removing various colour components from the camera allows for sharper and more light sensitive b&w photos... Also more b&w tones imo.
I do mostly B&W these days, using an Olympus OM-1. The ability to mix the colors in post enables some real subtlety that it would be hard to reproduce using optical filters on a pure B&W camera. I was lusting after this camera (and the Leica monochromes, though they are way too rich for my blood) before I got my OM-1; but I think I'm happier with what I've got.