I don't use the B/W mode of my Canon 20D. The reason is two fold. You can always do a better B/W rendition in Photoshop than in cameras, simply because the processor of your computer coupled with Photoshop's computing power is far greater than what can be built in a small camera chip.
But the main reason is that you'll get home with a perfect photo in B/W that you need to make color and you have very limited options. Imagine: A customer sees your great B/W and says, "Wow, that's great, I'll pay $10,000 for a color version for the cover of my magazine." Ooops, you only shot it in B/W! Too bad!