We use it in production. The web framework side of crystal is IMO more mature than what you find in Node.js, Python, etc. About a year ago we tried moving everything to Express because it's super easy to find fledgling js developers, but Express simply lacked most of the features we needed, even when supplemented by npm, whereas Amber did everything we needed literally out of the box.

Edit: Sails is not what it claims to be, and doesn't live up to the dream at all