I bought the "extra-small" size for a small backpack (16"H x 12"W x 8"D). I was hoping "extra-small" wasn't too small, but once I received it, I saw it was actually bigger than I expected, and plenty big for my needs. I used it on a recent vacation where it rained pretty good a couple times, and it kept my backpack and all contents (cameras, maps, etc.) dry. It was easy to carry and easy to put on and take off. I'm a little paranoid and was slightly worried about the cover slipping off the upper part of my backpack straps. So I actually used the cover upside-down, and wrapped its attached stuff-sack through a backpack strap at the top of my pack to make sure it didn't fall off. It seemed to work well that way for me. For the price, quality, and function, I'm very happy with this product.

Looks fine, good quality materials, no durability issues, but despite having the right size for my bag and it fitting on snug, i continued to have trouble getting it to STAY on my north face backpack even for relatively short walks. Every time I thought I'd finally figured out how to get it to stay on and not come off, suddenly it would come off. Until one day, I was walking in the rain, turned around, and it had fallen off at some point and I couldn't find it by backtracking. Just a very frustrating product. You need to realize that if it somehow comes off your bag it's just going to fall on the ground, the product has no way to safeguard against that (it could have a tie or something at least)

The product seems well made and is extremely pack-able but the size is difficult to determine. I ordered the small for my day pack which is WAY TOO BIG. At first I thought I got the wrong size but it said it was a small. Fortunately, I have other bags that I can use this on however, I would expect you to purchase and exchange to fit your needs.

My wife gave me this for my birthday, after I had longed for a reliable one for some time. I'll use it both for summer backpacking around Maine, and for cold weather hunting trips. The XS size is just right for my larger hunting pack, measuring 23" top to bottom. The rain cover is lightweight, fits in an attached pouch and the seams are carefully sealed. I have not used it in the rain yet (too dry here in Maine) but I am confident this will do the job nicely, much better than a garbage bag and a better fit, too!

I bought the medium size for my Sierra Designs Revival pack. It's a 65 liter/3,950 cu in pack. The medium rain cover fits it quite well. I read a few of the other reviews and one of them specifically mentioned how this rain cover can only cover the top of the pack up to your shoulder harnesses and if it's really raining hard you can get water down your back and possibly through your pack. Depending on your pack, this is possible. However, keep in mind that Mountainsmith is making a rain cover to apply to many types of packs/situations. The only rain packs I have seen which cover the packs exceptionally well are when they are designed and sold by the pack manufacturer and are model-specific. I don't even know how common that is at this point in time but when I have seen them they covered very well b/c they were customized to the pack. If you need some, albeit no perfect, rain protection this cover is as good as any out there in my opinion. If, when you try this cover out on your pack, you find that there will be a gap for heavy rain to penetrate and possibly get your gear wet, you may have to plan on having extra covered such as a tarp or a poncho you can drape over your back while wearing.