I use the one the came with my Kifaru G1 Longhunter.
One of my partners has a cool OR cover that has a hood you can pull over yourself...it nearly makes a poncho and keeps his stuff way drier than mine.
I purchased one for my Eberliestock pack last fall when I was hunting for a Mountain Goat. It was wet and nasty most of the time, and it kept my pack and gear dry. I think it is worth it to have in the pack even if it's not used. It can be used to cover the pack when it's sitting outside of the tent if you want more room inside at night, or you can use it to sit on in the snow or wet grass to keep your rear dry when glassing. All kinds of neat uses one can come up with and they don't weigh much.
I tried one a couple times. Either kept falling off or got torn up in the alders. So now I just put the important stuff in dry bags inside my pack and the rest can get wet if it makes it inside the pack.