I got the xxl for a osprey atmos 65 (medium) because of the measurements. i'm happy with the fit and performance of the raincover. however, the carrying sack isn't much use. a drawstring bag made out of the same material as the raincover would make a better choice than the velvet cloth used for the bag received with mine.

This is an excellent product; I cannot say that enough! I bike/walk to work and am worried about sudden rainstorms (an issue in my current location). I already had rain coats, but my Ibagbar Canvas Backpack Travel Bag Hiking Bag Camping Bag Rucksack kept getting wet even when using an umbrella, so I bought this guy. My bag says it needed a large, but I decided to get an XL just in case. It works perfectly! My bag has some shape/structure to it so the elastic edging the cover slides on easily and stays on. The cover also has what I believe to be two tie strings if you want to attach it more securely to your bag. The cover packs up small so it lives in my backpack in case of rain.

I also ran a triathlon on a rainy day and had no bags to keep my stuff dry at transition. Well, as a last minute idea I grabbed the rain cover; it surely wasn't going to work worse than nothing. I set up all my stuff at transition, but keep all my change of clothes stuffed into the rain cover and with the extra space and the elastic, I folded it over itself like a calzone, hooking the elastic around and under the stuff. It worked like a charm. The rain continued for about an hour, and when I made it back to my stuff it was completely dry!

This cover fits my REI Trail 25 pretty well. It keeps the pack dry and mostly stays on. The elastic feels very durable.

The missing feature is that there needs be a clip/carabiner/strap in order to attach the cover to the pack. As you can see in the photos, when the pack is not being worn, the cover fits securely on. However, when the pack is worn, the straps of the pack push the cover off the lip of my (and I surmise most) packs. My solution: use a binder clip at the top. Not elegant, but sufficient thus far. I would give this product five stars if I had confidence it would never fall off on its own.

I felt so smart whipping this out and covering my backpack when I encountered an unexpected rainstorm while temple hopping in Cambodia. It was seriously down pouring and while everyone else was scattering I was in waterproof gear head to toe including this cover over my backpack. It was key in keeping my electronic and camera gear dry. Also important that it's easy to just slip over my backpack in a hurry before anything gets drenched

Works like a charm, though I definitely recommend sizing up if you're close to the upper limit. I have a 28L backpack and tried a SM (15-30L) which was way too tight. The M (40-50L) fits great!