Picked up the largest one they had to cover my 3800ci backpack.  The one that comes with my pack is thin and barely covers my pack, so I was hoping this one would not only cover my pack but also my sleeping bag strapped to the outside of my pack.  Didn't quite do the job.  But, this cover works perfect on my backpack when nothing is strapped to the outside.  I also use one of these for my bail-out bag 3 day backpack and it easily covers that even with items strapped to the outside.

Happy that it's bigger and stronger than the one that came with my backpack, but not quite as big as I hoped.  1 star taken away for that, otherwise the cover does its job and is stronger than my other one.