I have used them in a chocolate milk stout and in a doppelbock. I usually add them to the secondary after letting them soak for a couple days in Vodka. Once they have soaked, I throw the whole thing in there (including the vodka) and let them sit for about a week or so then rack to the keg. The pieces sink to the bottom and are usually large enough that a bag is not necessary.
I used 3 oz for a 5 gallon batch of stout and while there was some chocolate aroma, it was not dominant. I used 4 oz for a 2.5 gallon batch of doppelbock and while it was definitely noticeable in the aroma, it was not overpowering.