If you have a more advanced camera, one that can take simultaneous RAW and JPEG images, you can set your camera up to take a b&w jpeg and a RAW colour photo — with one click.
Once you set it up, you can see the scene in b&w through the screen and, of course, colour through the eye level viewfinder.
I had never tried this until yesterday, and chose a cold, windy, bleak day for my first experiments.
I took about a dozen pairs of photos and uploaded them to the computer. I tweaked the jpeg monos in Lightroom and converted the RAW images to b&w in onOne's BW module. In each case I preferred the conversion to the original jpeg: usually by quite a lot.
In the following pair, the discrepancy was least, but I still got more from the conversion than the original. To see what I mean, compare both the sky and the shadows on the frozen pond. I think this conversion ...
... is clearly superior to this original.
While some of this may have been been a fluke due to whatever dials I chanced to hit in post processing, it also seems that when it happened with three different photos that it is best to convert in post processing than rely on the camera's b&w version.
However, the benefit is also being able to see the image in b&w when composing the photo as it is different than always seeing in colour. So, it could be worthwhile experimenting some more with this technique, preferably when the weather conditions are more amenable.