I've had problems getting that module to work properly with AWS ELB (though I'd kind of assumed the problem was with ELB), so I'm not sure how solid the support is even for that. It'd be nice to test it against nginx itself as a baseline.
We use proxy protocol ELB -> nginx in production at CoreOS for Quay.io, but we use the Tengine 2.1.0 fork of nginx for some other patches.
Yes this is important if you want to load balance http traffic at the TCP level and have the remote address in the back end app. Nginx already support decoding the proxy protocol but I wonder if with this new feature you can encode the remote address with proxy protocol.