I’ve just finished doing the Neo Ruby Koans. Mountains are again merely mountains. It’s an interesting way to get a sense of how the language works by looking at edge cases. If I have a knock on it, it’s that you spend more time reading and filling in the blanks than coding.
Unfortunately, it also only speaks to Ruby 1.8 and 1.9. Anything peculiar to the newer versions of Ruby will not be present. All in all, I recommend going through it. The more you know, the better you are. And I think Neo/Edgecase did a great service by making it free to the world.
With love and gratitude,