I like nginx, but it has way too much of a sacred cow treatment by the dev community. It has plenty of problems, the configuration is a psuedo-language that doesn't always make the right choices and is difficult to heavily customize, and I've gotten to it be -very- unstable under certain circumstances, including really bread-and-butter things like SSL caching. If there's a bug, you'll have a good old time debugging it's massive collection of C code. It's great, but it's not perfect.

Making nginx do custom things that you'll probably need to do in a serious environment (example: dynamically programmable SSL SNI) requires craxy mods and hacks that have only recently been made available (by third parties) and heavily reduce nginx's performance. Further, they only provide purgable proxy caching via their commercial version, which costs an exorbitant amount of money. The free purger, naturally, makes nginx lock up. I wouldn't mind chipping in a bit for nginx because I want to support their team any way, but at their current prices ($100/node/month or something like that) we simply can't afford it.

I realize this is not a popular opinion right now, but node.js is completely up to the task of running a reverse http proxy. They are basically (you likely won't notice the difference unless you're running the New York Times) competitive with nginx for performance, and as a tradeoff for an unnoticable slowdown you get a full, turing complete programming language to completely control the flow of your data. Nginx under the hood is just a reactor pattern with children that share a socket. Node.js has a cluster module that uses the exact same strategy. Mind you this is from someone that has done talks critical of reactor pattern scaling.

Also, if you have blocking I/O apps, it doesn't matter what you configure nginx to do, it's still going to lock up when someone DDoSes it with slow loris connections. Make your ruby app thread safe and use Rainbows! instead of Unicorn, or you're going to have a bad time.