Though the ability to arbitrarily create first class syntax is why Rails is Rails, my favorite use case is FizzBuzz as a property of Integers. YMMV.

One can be creative here, but at the end of the day, a lot of what is generated, is just a waste of time. People love to tinker. Then they move on, often before having written any documentation. This is also a curse in ruby - people hate writing documentation. There is a reason ruby lost to python - over-tinkering and dislike of documentation are two smaller reasons.