nginx is an excellent high performance webserver, which is something F5 don't really have. It is (or was, at least) at best a mediocre load balancer, with all sorts of limitations at the sort of scales you'd usually use an F5 box for. While it's probably fair to wonder how much more work will be put into the LB feature-set of nginx, I hope that this will be counterbalanced by increased funding for the core feature of being a superlative webserver.
NGinx is first and foremost a web server with excellent controls around caching. Haproxy is a load balancer with some caching as of version 1.9. F5 has always provided a web server in their load balancer via apache, but that is for management of the device itself. It can also serve up static web pages (primarily meant for error and landing pages).