It’s ok, has a couple of big vents under the arms to let some air out. I doubt it would be as breathable as some of the others mentioned at the higher costs though. I guess it is more of the ‘plasticy’ type material than a ‘softshell’ material.
I haven’t really used it for prolonged periods though maybe about half an hour tops, it only comes out in an emergency. For me it’s mainly my bare arms that get sweaty inside it from directly touching the material I guess. It’s fine for the job I wanted it to do, and it folds up really small (not much bigger than a fist) to carry around easily.