This seems like it's a lot of overhead for what amounts, in terms of capability, to a reimplementation of greasemonkey. It also makes you implement finer controls on execution by url yourself, whereas greasemonkey has them in its syntax. The author states,

"GreaseMonkey user scripts are great, but you need to publish them somewhere and re-publish after making modifications. With dotjs, just add or edit files in ~/.js."

but this caveat is just as strong for files you maintain outside of your browser, and some browsers' implementations of userscripts/greasemonkeylikes actually have a similar filesystem-based model for managing scripts already.

While respectable, I had hoped to be more impressed by a tool that beckons me to "hack the web".