just traded my nikon 12-24mm in for this 17-40mm. im sad to see the old one go, but happy for new things. just bought the body too but havent had a chance to use it.
i traded it in at my local camera shop, they gave me 500 for it which was alot less than i wanted but i had no offers locally.
the end peice is called a lens hood, it prevents light rays from street lights and the sun from making flares on your photo. it also acts as a protector if you drop your camera.