Oh yes, lots--works fantastically well.
http://shoptalkapp.com is nginx -> haproxy -> (cherrypy/diesel)
http://shoptalkapp.com is nginx -> haproxy -> (cherrypy/diesel)
I.e., if Host header is X, and Ip is Y, but no cookie was provided, then use pool... with this balancing scheme.
nginx's proxy setup out of the box is useful, but I wouldn't consider it on par with things like ZXTM or a Big IP when it comes to robust balancing/fallback patterns and flexible routing rules.
If there's a module that provides all that, I'm happy to hear about it--I haven't found it yet.
So I can't do my fancy HAProxy routing rules on HTTP requests HAProxy can't read--it would be reduced to a simple TCP-level balancer then.
Basically, nginx is there for ssl, gzip, caching (redis/memcache mod), and all those http-plugin-goodness things. Simple proxy directly to HAProxy on loopback, which specializes in load balancing, traffic routing, failover, etc. Then, finally, the application code on N nodes behind HAProxy.