While most customers want color prints, I'm partial to black & white. It's not to look nostalgic. It's just that we can modify the shades of grey in ways you can't with color, to affect how people look at the image, what is the focus, what's important. For example, look a picture that includes the sky. In a color image it's perhaps blue. But in greyscale what happens to blue? By adjusting the filter the blue can be sorta white, or very grey, or even approach black. None are "false." There is no "real" grey shade that equals blue. So you can decide, based on the specifics of your image, how you'd like the blue sky to look. One reason Ansel Adams' prints are so dramatic is that he used a red filter to make the blue skies dark, which makes the rocks (and moon) stand out in certain images. Here's the same image with different filters on the blue sky:
This is not only the case with blue, but with all colors -- although it's a particularly striking effect with blues and also with greens (sometimes a field of grass should be dark, to make it disappear, and sometimes light, to highlight objects in the grass). Where this is particularly important is when the color (blue or green or whatever) is distracting and draws your attention away from the subject. Playing with the greyscale versions of the colors helps you control the attention of the viewer.
I like that shifting an image to monochrome is a reminder that in actuality all photography is surreal, even when it looks "natural" — we experience the world in a large and continuous way: this abstract freezing of a slice of visual space, for a fraction of a second... and the colors we see with our eyes are very different from what can be captured... no, photos are abstractions and b&w is a gentle reminder of that.
IF YOU WANT TO USE COLOR
Be mindful of the colors in your image. Does it matter to the story you're telling? Is it distracting from the subject or is it part of the subject? Lots of colors give your eyes lots of targets to focus on, but this can be a detriment to a powerful image. A good rule of thumb: examine a photo in greyscale, think about where your eye goes, and how the composition is working; and then ask if it's *better* with color.
Finally, be wary of over-saturating the colors: too much richness can be distracting and even grotesque. It's like too much salt when you're cooking: it definitely adds taste and makes other ingredients taste better, but too much can be a bit sickening. Sometimes subduing the colors produces a stronger result. Adjusting saturation of an image, both a little up and a little down, are often interesting ways to refine a print.