The course designer leaves major bugs for weeks that would have taken 5 seconds to fix. This thing is definitely not ready for prime time, unless you are a glutton for punishment or enjoy bug detection work.
Brain dead easy (if you don't count all the bugs). It may not be a good way to learn because it takes such tiny steps each way. I'm not sure.
There are some seriously bad practices being taught in this class that defeat the reason why CSS was created and will result in giant messes once you use this advice on real world websites. If you aren't already a skilled software engineer/architect, google something like "Best practices CSS" and you'll learn what is important and what parts of this course to ignore/avoid/reject.
It also has bugs. One common one is because of a dumb architectural error in their system - they apparently don't read your actual code, but instead run it and then attempt to query your system to figure out it ran right. This makes their system VERY browser/browser-setup dependent. When you have annoying bugs, and you don't know its a bug but think its because you didn't do something right, you can waste significant time tracking down their bugs. Check the forums, because it is likely a bug that others are complaining about. So when you get an error message saying you did something wrong, if it isn't obvious, it could very well be that they had a bug, not you.
Minor nit for Codacademy (unless this is only a problem in this course): I like to be able to scroll left and right through the "pages" of this course, much like a book, with a simple arrow icon. Instead I have to go to the top left, click on a small button that displays a popup menu, and then click once more on the previous or next item. This is more tedious than a simpler more intuitive interface.
From time to time, the display of what you are doing is wrong, out-of-date, empty. Another sign of bugs or the course builder choosing an architectural design that relied on things more complex than they were able to deal with.