Is probably one of the better learning sources I have ever found. Takes a game approach to having you do many basic and increasingly complex tasks. It essentially gets you experience, and experiences I usually am not exposing myself to. In the process you learn to use new tools, and the same tools differently (hopefully more effectively). Focuses on the core GNU tools, and their incredible versatility.
Where this succeeds that a standard bash/linux/CLI curriculum fails is getting you tangible experience with a basic reason as to why you'd do what you doing, and a vague role playing notion that frames the steps in a problem solving capacity. You are given a problem and expected you will learn the steps to overcome the problem. The steps are small enough that you actually do feel you are building a tool-set, rather than just finding several one time solutions. You will be Googling, but for minor pieces at a time. Like any good learning resource there is a fair degree of repetition between challenges, but this helps retention.
Seriously within a few hours of doing the first challenges you will begin to see the CLI as incredibly more useful than you have prior (assuming you are a beginner/moderate Linux user).
an example:
The password for the next level is stored in the only human-readable file in the inhere directory
You are tying to elevate your SSH sessions increasingly and to do so you need to get the passwords. In this case there are a list of files in the directory mentioned. Only one of them is readable. Find it, use that password to elevate your SSH session. Don't worry when you get to it you will have 90% of the info you need, but you will need to google/learn the other 10%. The next step will build off of that.