There are some good Gygax adventures in there. Most notably "Necropolis", which is a high level adventure. This releasing of high level adventures with brand new systems is something he did with D&D too (G series). "High Time at the Flying Pig", or something similarly named, as well as a slaving ship adventure are both pretty nice. The magazines included some interesting one's too, but those are harder to find.
The "Epic of Aerth" is an excellent setting book and interesting to read, especially if you are interested in his other campaign settings like Greyhawk's Oerth.
The game rules are a skill-based cluster system and extraordinary in their bulk. The descriptions are interesting, but the system is probably too much for most groups to want to keep up with.
Both the main Mythus book and the Mythus Magic book are good for inspiration though. But the Bestiary isn't. It focuses on mundane animals, rather than mythical and magical monsters.
Can someone talk about Necropolis? I think I've seen it converted to LJ, was it?
Necromancer Games converted it to 3.5... or maybe 3.0 just before the switch in 2003.
Necropolis is an adventure in an alternate earth Egypt in the middle of the pyramid building dynasties. The adventure ramps up from about mid-levels to very high levels with the understanding the PCs could or would return throughout the campaign. It starts with a large player handout introduction in true high Gygaxian language and in a small rural town. It progressively gets bigger and harder with different steps along the way. In short, a minor cult leads to a major temple, behind which is a maze-like crevasse of burial crypts.
The real centerpiece of the module is the final crypt, which is as large and deadly as anything found in S1. I would say it is not as unfair in design for the PCs exploring it, but it includes many wrong turns and devious false endings. The Egyptian mythos is clearly something Gygax was proficient in writing for and he published at least two novels of the same for the game's book publishing side as well. Paizo republished his novels only a couple or so years ago, so they should be available to supplement this adventure, if one were so inclined.