I did 8 oz of cocoa powder for my chocolate oatmeal stout, added it the last 10 minutes of the boil. I also added 2 oz of vanilla extract before kegging it, and a total of about 1 lb of lactose. Unfortunately I think it got a little infected or I did something wrong, since it has a slightly acidic tart taste to it. It's either due to the alkalinity of the cocoa powder, the amount of lactose, or an infection. I figure since no one else who has brewed with cocoa powder or lactose mention any odd tastes, must have gotten infected... Still drinkable though, and I just tell myself I was going for tangy taste...
The 8 oz of cocoa adds a nice chocolatey smell. The smell is more powerful than the taste, but it does have a chocolatey taste (comes out more as the beer warms up). The chocolate flavor is pretty subtle though, it adds a complex bitterness to the beer but doesn't beat you over the head with a sweet chocolate flavor.