I built a production app recently, Rails 5 (beta -- surprisingly stable) + Heroku + Passenger, separate React + react-router frontend. It's been fantastic so far -- has handled a decently sized production load, including some pretty heavy ActionCable usage, with no problems. There's a few tricks to getting it working on Heroku though; I've been meaning to write a blog post about it but haven't found the time.

Basecamp is already using it in production for what it's worth.

I made a new internal company project with it. Nothing really changed or broke since 4.2 but it seems faster.

I still haven't used ActiveCable though.

I've been running it in production since the first days of the year with a small new application. No problems whatsoever writing, testing, deploying and running it. I don't expect particular problems upgrading Rails 4.2 applications.

I didn't use ActionCable because my application doesn't need it. I experimented with it since the first version, which I plugged into Rails 4.2. It had some quirks that were fixed. It works well. There is obviously code to write in the client to handle the websocket but it's pretty much standard boilerplate. I expect some gem will appear implementing it in some clever helper, except the code we really have to write.

While I haven't moved into production yet; nothing broke when I migrated my Rails 4.2 application - just had to tweak the few things to new conventions.

https://www.terminal49.com/ is running on Rails 5 (beta2) API on production. Using ActionCable + Ember + JsonAPI