Very lightweight and feels durable
I read the reviews regarding sizing and went with a large. I typically wear medium running shirts and some large tri shirts. I have 5'10 with about a 42" chest and it fits well. I could probably survive if it was a little smaller, but a medium would likely be too restrictive.
The jacket itself is great. It packs down small and weighs next to nothing. When squeezed tightly, it is about the same size as my fist. Despite the light weight, the material feels very durable. I do not feel like it is going to rip if I pull it too hard or tear a seam trying to take it off quickly. One thing I really like is the pocket which you store the jacket in has a double zipper (handle on both sides). You can close it all the way without having to tuck the zipper in.
I am a little confused by the buckle strap shown in the pictures. If you snap it together like it is shown, it pulls the flap away from the zipper. If you secure it so that the flap goes over the zipper (like you would on a jacket), then the clasp is upside down. Not a big deal, just a curious design.