I've seen this come up a few times, and benerved myself to ask the really stupid question: what is this for? Why am I supposed to want a process boundary between my HTTP stack and my applications, instead of running them inproc with mod_whatever? If it's a load balancer, how is it better than using a NAT to forward TCP streams and letting the app servers handle HTTP?
For one thing, mod_whatever may not exist for whatever hot new language you want to use. ZeroMQ already has bindings for most languages people might want to use (including weird ones) and there are only like 16 C functions plus some constant declarations, so bindings are easy to make. And once you have ZeroMQ support, Mongrel2 can talk to you. That may not be a killer feature for you, but it's really nice.
Plus, everything in the design of Mongrel2 just seems to make sense. I'm not sure how to describe it; Mongrel2 smells good.
