>> How well does it breathe?
All I've had a chance to do is walk the dog but I'm pretty sure that it won't breathe well at all. But that's something I knew and expected.
That said, I think it would be really easy to build in more venting options, like pitzips. I made the jacket in one evening and didn't have extra zippers but, if I had, I probably would have put them in. It's right along a seam, so not a difficult option.
I also over-sized it a bit to allow for layers and I think the looseness helps in circulating the air around inside. But I don't think you'd want extra fabric flapping around for cycling. If I wanted a more trim fit, I think I might consider sewing in breathable side panels, maybe out of a stretchy fabric.
In the end, my priority was for lightest weight and waterproofness. For the type of hiking it's intended for, I'm not expecting to wear it all day.
>> looks like a good way to use up all those silnylon scraps I have hanging about ...
Yes, in fact, what I had was 2 yards of a seconds piece that had defective spots in it that I had to cut around. Without those spots, I probably could have cut it from a 1-1/3 yard piece.