Update — 3 months later.
I have been using unicorn in production for 3 months, and I have been very pleased. I use 4 unicorn workers per dyno.
One thing you do need to keep an eye out for is memory consumption and leakage. In effect instead of having 512MB of memory per dyno — you have that divided by the number of heroku workers. But keeping that in mind — it has been a great cost saver