I'm wondering what do you guys think of Koa.js? I find it really interesting and I've been meaning to give it a test run since it was announced a few months ago. Using generators instead of promises sounds seducing. I believe a blocking concern is that you have to use Node 0.11 to use Harmony features. Using an unstable version of Node to deploy application in production is worrysome, but with 0.12 being along the corner, it seems to be a vanishing concern. So, what's your take on Koa? I would be interested to know your opinions. If you had to start a new project from scratch, without external contraints would you use it instead of Express 4?