I've tried it out a couple of times, and it is included in my binder full of random tables for improvisational game mastery. At its core, Mythic is a decision-making system that takes over the job of adjudicating your tabletop game. Think of it like a more advanced version of the old magic eight ball toy, answering any yes or no question you might pose. It also provides ways to generate keywords that can inspire an infinite number of random encounters.
Finally, it structures the game in a way that makes the master-less adventure flow smoothly. By breaking your game into a scene to scene structure and assigning each scene a "Chaos Level," the system allows you to play out a series of engagements that tend to escalate in difficulty. Chaos levels increase the likelihood that your questions will be met with answers of 'yes.' Since those are typically questions like "Is there a booby trap in this room?" or "Is the duke angry with us?" you can see how things can get very interesting very quickly.
Overall, it is a fun and unique tool that I would recommend to any game master for their own toolbox. It can be used in conjunction with any system (Though it works best with simple systems that are less combat-oriented), and could also be used as a tool to help aid a game master with decision-making and inspiration in general. Check it out now on DrivethruRPG.