This is the addon I will buy for almost every project, since it usually saves days of works and the associated bugs. Before this I used Block Designer from the same author, but this one is better if you code - it's faster and since it's all text based, your block definition are under version control and part of your packages, you can easily copy/paste or better build functions to build block definition or factor the repetitive parts. It costs less than a hundred euros and it saves hours or even days of headaches and repetitive and boring coding.
I only have two negatives with the addon : it should be part of the core of Concrete5 and it would be nice to have a few more built-in types.