You shooting raw or jpeg?
Jpeg - can't comment too much as I don't do it but I'd guess it's going to be like hitting the monochrome button in photoshop, no adjustment, no control just a desaturated colour image.
Raw - depends on what camera brand and software you use. I shoot Nikon and use Adobe bridge and photoshop, I spent ages using Picture Control, getting a nice looking b&w image in camera but then realised that when I download the images bridge/photoshop don't see the Picture Control settings so I just get the basic colour raw file.
I now have a basic b&w shooting bank which just gives me a b&w preview - I find just having the preview in b&w can be useful as I work, it helps me envision what I'm doing sort of thing - and I do the onversion in Photoshop.
Quite often il make a b&w conversion layer and toggle it on and off and decide the colour image looks better, an option I wouldn't have had if I'd only recorded a b&w image.