I've played it a few times and started when I had just the main Dangerous Journeys book. I ran a short campaign where two Tripolitanians become crew members on a interdimensional ship called The Winged Pig. (The name is of course from the intro adventure from the Mythus Prime section of the DJ book, and it was the intro adventure of that campaign as well.) For the other dimensions I used the tables for different worlds/dimensions from the AD&D 1st ed Manual of the Planes. I just rolled as they travelled and improved from there.
The rules are detailed yes, but this usually only comes up during character and NPC creation. It took me and a friend 24 hours to create the first PC we ever did with the system. When we did the characters for the campaign above we managed in 5-6 hours. But doing NPC and creatures takes so much time for me as a DM so for that reason alone I won't play it anymore. Like others, I mine it for ideas though.
Compared to D&D 3E it is a very different game. Where 3e has Knowledge [knowledge area here] DJ has them all written out. And you better read them all to see if you get Heka and any other bonuses from them, whether you should specialize or not. You have varying degrees of sensitive areas of the body which can take hits. And in what seems a throwback, elfs and dwarfs are classes, whereas humans can be one of...a lot of classes.
For those that aren't used to Gygax' game writing (as opposed to his novels) you'll probably have a real reading challenge ahead of you. And as other shave pointed out, some additional editing wouldn't have hurt at all.
AD&D 2nd edition is my fantasy game of choice and I take ideas, and some rules from DJ to supplement the experience, along with AD&D 1st ed and 3E. I should go and buy Lejendary Adventures to get another source of idea mining but I think I'll go for the Gygaxian Fantasy books first.
If you want to try it out, try Mythus prime first with the adventure that comes with it, and if you feel like you're up to it try Mythus with the other adventure.