I ended up ordering the XL because I wanted to have extra room to cover anything attached to the outside of my pack. I got caught in a couple rain storms last summer while backpacking and this raincover was fantastic. It had plenty of room to cover my fully loaded pack and had a cinch strap to make it snug from my neck to my waist, and an additional strap across the middle to bring in the sides. I also love that the carry bag is attached to the bottom, so you never have to worry about losing it.
It is super light weight (similar to the thickness of a backpacking tent) and looks pretty stylish for a raincover. I assume you'll have to be careful putting your pack on the ground while this is attached due to the thickness of the material, but if you take good care of it, I'm sure it will take good care of you. If you've got an Osprey pack, this cover is a no brainer. If you have a different brand, I'd still recommend it as a quality raincover.
I have 3 of these for various Osprey packs. They pack down well, don't weigh too much, keep your pack dry when it rains, and are reasonably durable for how little they weigh.