The tutorial videos in this course are longer than many of the other Treehouse courses, yet still remain easy to follow and very engaging. The material is presented at a reasonable pace and everything is clearly explained.
This course will give you some idea of what you can do with D3.js, however it doesn't cover many of the useful functionalities. You will have to seek out other resources to complement your learning in order to fully utilize D3.js.
The free only version is all the material placed on one page. It's quite a bit difficult to follow as the material is quite tightly placed and you have to scroll back to the top any time you want to refer to the table of contents.
The tutorials are written in a way that makes them clear and informative without being bulky. Due to the tutorials being example-heavy, you are able to see the concepts learned in code after every step.
These tutorials are a great place for a beginner to start, but if you want to do more advanced things with the library then you'll have to continue with another resource. However Scott Murray has a book, "Interactive Data Visualization for the Web" that extends his online tutorials.
D3js.org provides a long list of examples demonstrating the library in use, and the code for those examples. D3js.org is also where the documentation for the library can be found (such as a list of all the methods available).
Though D3js.org provides some tutorials, the documentation is better suited for developers who have already been introduced to the library and need to reference the official documentation. Someone new to D3.js will most likely find themselves unsure where to start.