Beyond what's good or bad, it's a fantastic reference for all things fantasy. It's like a weird encyclopedia with useful information littered throughout. I can't think of a fantasy campaign I've run Mythus where those books haven't come out for something. Need an idea for a spell? Mythus Magic has 1400+ of them. Need to flesh out a skill or specialization? Chances are it's in there. Demons? Spirits? Elementaries? You know you want to know what those are.
I love the game, warts and all. While all the rest of you get all nostalgic for B/X D&D, I'm looking over my Mythus books thinking of how to convince my group to give it a whirl.
The good: it's just got lots of cool ideas mixed in there. It's a percentile based system that gives you the D&D game experience. Not the gritty realism of Runequest, but more pulpy with some big dice rolls. Characters are cometent, well rounded and bursting with possibilities.
The bad: character creation is...a slog. Especially when you turn on all the bells and whistles. The book tells you all these bells and whistles are optional, but doesn't do much to help you decide what you want or don't want in your game. Spell casters are an even bigger slog. On top of that, the game was edited by blind monkeys. So if your going to play, your going to need to track down the first couple of issues of Mythic Masters Magazine to make things work.
Or you could do what I finally did: start with the basic game (Mythus Prime) and add advanced options until you get something that feels right. That's where we had the most success with the game.
If you do get it, PM me and I'll point you toward some enthusiasts of the game who can help get you going. But even if you never intend to play, the books are worth it for the content alone.
Tom