I'm actually using it in a small side-project I implemented a couple of weeks ago. (http://playas.io) It uses Rails 4 [0], JRuby 1.7.4, postgresql with full text search, one dyno and 10 workers.
It is not the first time I'm using JRuby and Puma, I must say I'm pretty happy with it.
I had enough with the official documentation, just tweaking the DB pool to fit in the Rediscloud free version constraints. [1]
[1] http://blog.steveklabnik.com/posts/2013-02-24-using-puma-on-...
[2] http://www.subelsky.com/2012/10/setting-up-puma-rails-on-her...
[3] https://devcenter.heroku.com/articles/rails-unicorn