I've got an earlier version of this. As you say, it is very boil in the bag, so even the easiest of efforts gets a sweat on. I rode 3 miles from the station to the office yesterday and it kept me nice and dry but I was fairly sweaty when I got to the office and I just pootled along.
I have the other decathlon one
Trialled it last week and it kept me dry, but was quite sweaty and a bit flappy in the wind.