I'm using it for the Common Lisp exercises. The command line client is nice, keeping you from having to upload your code through a browser or copy pasting.

Basically you are given a set of unit tests and you have to make them pass, so it's good practice for TDD without havingto write the tests. The exercises so far are pretty basic being <10 liners, but it's a fun way to practice your coding.

I did a few exercises a while back in the Python, Ruby, and JS modules. Feedback from other users seems minimal; I only received one comment from another user and one automated comment. I haven't commented on others' code, either, but still, it was good practice.