I am a big fan of black & white images. But when to use B&W effectively I think depends largely on the subject matter and what the photographer is trying to accomplish. Some images naturally lend themselves well to the B&W format, while others might not work as well in monochrome. And some may work well in both mediums.
There are times when I see certain subject matter, and I automatically know that it will end up as a B&W image. But sometimes I will just quickly desaturate an image in Photoshop just to see how it looks.
A good example of a gallery that I knew beforehand would work well in B&W, is the ghost town of Bodie http://www.pbase.com/olybob/bodie_bw. I wanted the images to really bring the viewer back in time and also portray the somewhat spooky nature of the town as it stands. Then a few years later I decided to go back, with the intention of creating a color gallery of Bodie http://www.pbase.com/olybob/bodie__color. I think both formats work well, but B&W seems to suit this subject better and achieves more of the feeling I was looking for.
Two examples, from each gallery gallery:
Although both B&W and color seem to work here, I think it is the B&W image that really brings back feelings of the old wild west.
Bob
http://www.pbase.com/olybob