Learning about algorithms is far more difficult and far more important than the mechanics of writing code, which is the focus of sites like Codecademy. I believe that there are certain people whom have minds that can naturally pick up algorithms and break down problems into the different steps needed to solve them. Codecademy will give these individuals most of what they need in order to develop proficiency in writing code.

However, these people are not the majority. When I TA'd for an introduction to Computer Science course, most of student's difficulty, especially on more difficult assignments, was not syntactic and getting a program to compile or run but rather conceptualizing and implementing the intermediate tasks that the program needed to accomplish in order to produce the final result. In my opinion, this is the most difficult part of writing anything beyond the most basic piece of software and the area which online programming courses are the least helpful.

That being said, I think that Codecademy and similar sites are very helpful of learning the syntax of a particular language which, of course, is the first step to being able to write code.