On a talk from Daniel Stenberg earlier this year he told that over 60% of the clients already support HTTP/2. So there is a real incentive to start supporting it as it nearly always is faster and costs less connections for the server.

Eyeballing the stats at http://caniuse.com/#search=http2 I think it's more like 85% of clients support it.

I installed HTTP/2 indicator extension for my browser and I am very surprised to see how many sites are already using HTTP/2. It's easily close to a third of the sites I use frequently.

I wonder if those are mostly sites using Cloudflare and similar, which supports HTTP/2 regardless of the upstream server's support.