I had a Extang on my zr2, I loved that cover. it was built well, stayed tight and kept a lot of water out. I should have bought one for the Sierra, but I don't see the one I have lasting that much longer, so I will probably buy one.
My rents bought the roll n lock for their dodge, its a beast of a cover. We can still load all the kayaks in it how we normally did without a cover and its fine. Only thing I don't like about it is all the funny sounds it makes as you drive along, granted that trucks suspension is way stiffer then these half tons, you can hear it.