I bought this cover to fit a Kelty Trekker 2950 External Frame pack. The cover barely fit - be careful if you are ordering for a larger pack like the Tioga. It rained lightly while we hiked and the cover kept my pack dry. It is light weight and is probably a good choice if you want to minimize your weight carried.
I wish that it rained more heavily - but I will probably check the waterproof carefully with a hose before hiking again.
I bought this pack cover for my husband's backpack. I didn't have his backpack with me when I ordered the cover, but it fits the backpack perfectly, and kept everything dry even during a rainy week on the trail.