Having done a couple of their courses without paying: You are expected to complete the project in steps they define (so for their Redis project, step 1 is to b

Having done a couple of their courses without paying: You are expected to complete the project in steps they define (so for their Redis project, step 1 is to bind to a port, step 2 is to respond to a PING command, etc). If you choose not to pay, you can only complete one step per day, even if you submit code which would pass future steps. This can be quite frustrating, since each step is often very simple, and IMO discourages producing a well-architected solution which anticipates future requirements, as you're left waiting 24 hours to press the submit button for code you've already written. Still: It's free, and the restricted progress forced me to not use it for procrastination purposes, so there's that.