I really enjoy codecademy (I am currently at 930 points and 47 badges). I find its a great way to learn and practice new concepts with a much smaller barrier to entry and instant feedback. It is by no means perfect. There are a few issues that come to mind:
1. Because of the way the code validations are written there tends to be only one or two ways to pass a lesson. This can lead to frustration when a student writes a piece of code that performs the task correctly but because it doesn't validate correctly he/she doesn't pass.
2. I found (for me) that the pace is actually to slow. I have cruised through all the lessons and have participated in beta testing some of the more advanced javascript modules (i.e. introduction to JSON). It would be nice if there was more advanced content to move on to but this should be corrected as time goes by and more content is created.
3. General spell check and editing could be better
4. Currently there is no content that talks about the full stack of technologies in a web application. This leaves the student studying small siloed pieces such javascript/html/css on their own without a view of the bigger picture. How does a web server fit in? How do I communicate with a database? Why is there no talk of MVC or REST? Basic coding skills are great but without a comprehensive picture of an application they are pretty useless.
Even with these issues codecademy has been a fantastic resource for me. I have learned a ton and have gone on to start teaching myself more advanced topics and have been learning different libraries such as backbone.js, node.js and d3.js. Without codecademy I would not have been able to get through the documentation on these libraries. One of the most valuable things that codecademy has taught me is to not be intimidated by technology. From the outside the programming world can seem like a pretty intimidating place. Codecademy makes that first step a little easier...