I don't often use selective desaturation. It's an oft over-used effect that can easily come across as cheesy. Every once in a rare while, however, it feels like just the right thing to make a particular image or set of images work.
I wanted some of this set of Catmaexx to be in black & white, but I really didn't like the way the tulle looked after converting the image to grayscale. Even after a bunch of experimenting, I just didn't love the way her body looked through the material when it was just black and white. The original color versions were nice, but I felt like they needed something to make them pop off the page a little.
After some experimentation, I found I really liked these shots using selective desaturation, leaving blue fully saturated. I liked it so much, in fact, that I created a preset from it and did a whole series of them.