I can't say that I know it, or that I have used it. If I had to, I would say that I poked around CodeCademy to see what all the fuss was about. And that is what I have been hearing, fuss. And that doesn't always translate to quality. While, it seems decent for teaching beginners but In all honesty I prefer just reading up on the documentation and following written tutorials if I am trying to understand a new technology or language.
Yet, CodeCademy was making some waves and I do tend to checkout the source of those waves. What I kept hearing, where people say that it was pretty good and easy to use. What I saw though was that you have to really make it a point to understand what's going on.
I watched somebody use the Python course, and they were pleased at their pace and with everything that they "learned". When I talked to them later they were about halfway through the course, yet they hadn't grasped everything leading up to that point. This only became obvious when they tried moving on to the next exercise and they were completely lost.
The basic foundation was simply not there.
From my exposure I would say that CodeCademy is really only good to give people a taste of programming.
It will be hard to build anything past a simple static website after completing most of it. In the end I feel like it is a great way to get my feet wet in new technologies and may ignite a few peoples interest in programming.
As far as I can tell, the exercises are short and simple, yet that is rarely how programming works.