I believe it has been Google all along, I guess when you do things at their scale this matters, but indeed I can't see any average dev rushing to support it.
It depends. I have a site with a lot of assets (about 70 images) and using SPDY made it 30% faster than plain HTTP thanks to better pipelining.
It's lower overhead for the server, too.
Do you know how much lower overhead?
Well, it's not just the front end; it's the load balancer and whatever other infrastructure, since it packs things into a single connection more effectively. For hn, not an issue (since it's a single nginx in front of a single threaded arc process).
I believe some pron sites use it. The benefit increases with pages which have a lot of assets, which applies particularly well to typical pron sites with their multitude of images per page.