Tornado (http://www.tornadoweb.org/) can deliver long-polling with a pretty small amount of work as well, and I appreciate the simple, lightweight design of the framework itself.
Sure, but Gevent monkey patches the stdlib and lets the programmer write code in a synchronous fashion which actually gets executed asynchronously. That, IMHO is a big win since it's easy to get stuck in the callback hell in Tornado. I have used Tornado in production and it's an awesome library/framework/server but things do get complicated when the app becomes complex.
I'm not as experienced with Tornado, but just feel like throwing in that callbacks in a language without non shitty anonymous functions seems like it would be miserable.