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Pros
Pro Updated often
The gallery is updated frequently with improvements and new examples.
Pro A lot of visual appealing examples including source code
There are hundreds of examples to look at, including the source code. This will give you a great idea of what D3.js is capable of.
Cons
Con Examples only
The D3 gallery isn't a tutorial, it only shows examples of graphs created with D3.
For someone completely new to D3, this would likely be an overwhelming place to start.