The best part of Ex3 is that it has a combat system that works pretty well and uses concepts fairly new to the world of tabletop RPGs (for example, your turn order Initiative is also your combat momentum, changing as you attack or get attacked), and a social conflict system that's mechanically engaging without just being a mirror of the combat system (you have to discover the Intimacies of people and use them as leverage to persuade them to do things).
The worst part of Ex3 is that it's got a lot of "natural language" that never went through the hands of a competent editor, so there's lot of fundamental stuff that any game using it would need to houserule in short order, like "what's the exact order of resolution for attacks" (important for some effects), "how does a counterattack resolve" (there are literally no rules given for it), "can Dogstar Ruminations be defended against in any way" (as written it's just 'rocks fall and a city dies'), and so on.
It's also worth noting that it implicitly changes the setting at the ground level a fair bit from previous editions by getting rid of mortal magical martial artists, while at the same time making mortal sorcerers much more powerful than they were, as they can now control demons (previously restricted only to Exalts). It's a tilt away from wuxia and in the direction of Conan.