This is a technique some of you might be interested in using on selective photographs. I find it works exceptionally well when working with rusty stuff, like these cars. While the technique can be done just using the B&W Adjustment in Photoshop, I tend to use that first so I can push certain colors up and down the color spectrum as I wish, but change the blend mode to luminosity. Then, take it into Silver Efex Pro 2 and generally use the neutral setting and whatever filter I might chose (on this one it was the yellow filter), adjust the brightness, contrast and structure as desired and again, change the blend mode to Luminosity. Adding a mask, I can deftly adjust the degree of "grunge" I want either by painting out or lowering the opacity, or both.
This is a fun tool but as Frank pointed out, it doesn't take much to go over the edge. That said, I always save these originals in layered files so I can go in and adjust as needed.