Javadoc used to drive my productivity. It is quite frustrating sometimes with Rails that more often than not I can't find good api docs.
Rails and various Ruby gems have very good tutorials, but sometimes no api docs at all.. Tutorials are great for getting started, but you're in trouble when when you want to do something slightly unusual.
Slowly I'm realising that to be really good with Rails you need to look into the source code of libs you're using to get a better grasp.
As an example, I wanted to find out if it was possible to do something with ActiveRecord.to_json that I haven't seen in any tutorials. Spent 10 minutes searching for docs and then looking at the code.. which in Java would have been done in 20s.
Having said that, I really like Rails and enjoy a lot of productivity gains. I just wish it had better api docs.