The rules of Mythus are extremely complicated when taken as a whole, but (like AD&D) they're also very modular, so you can pretty much ignore vast swaths of them in play without really negatively impacting those sections you are using. Mythus -- like AD&D but even moreso -- attempted to be an encyclopedic ruleset, that could be used to cover just about anything, and in (IMO) characteristically "old school" fashion did so not by abstracting everything out to a single universal resolution mechanic (where, say, a swordfight, a seduction, and baking a cake are all resolved exactly the same way under the rules) but by piling on detailed and individualized subsystems -- so if your character has the "Buffoonery" skill, there's a couple pages of special rules saying how it's used; if he doesn't have that skill, those pages can be ignored.
Generating characters is the hardest part of Mythus -- it involves many steps, lots of math, and interdependent variables, and even for a simple character (a "fighter" or "thief" type) is likely to take several hours and have a character sheet filled on 2 sides; a spellcaster (and especially a "full practitioner") will take even longer and fill up even more space, as the character may well begin the game knowing 100+ spells. Which brings up another point, that starting Mythus characters are equivalent to at least mid if not high-level AD&D characters -- a baseline starting character is probably about as capable (in terms of combat ability, skills, hit points, wealth, etc.) as a 6th level or so AD&D character, and a full practitioner mage or priest is more like about a 10-12th level AD&D character. So while it will probably take a full session (or more) to generate characters, unless the players really screw up it's not likely they'll have to do so very often -- the campaign I ran lasted for about 15 months and saw 2 PC deaths (at least one of which was really more of a suicide).
What's probably not apparent about Mythus from reading the rulebooks is that, once the characters have been generated, in play it's almost exactly like high-level AD&D, to the point that running it almost feels like second-nature. You're rolling different dice, and everything has different names, but as far as what you're doing it's remarkably similar -- each encounter begins with a surprise check, initiative is re-rolled every combat round, characters generally get one action per round but high-level and quick ones might get 2 or 3 attacks in, magic is in the form of individual spells that take time to cast and can be interrupted, combat is resolved by rolling to hit and then rolling damage, characters targeted by spells get saving throws, etc. If you can play AD&D, you can play Mythus (and even if you're a spellcaster it's just a matter or reading the individual spell descriptions -- just like AD&D).
The biggest hurdle is definitely the terminology -- Mythus pretty much completely eschews standard rpg terminology (with Game Master being about the only holdover -- and even that got changed to Journey Master later on) -- characters are Personae (Heroic Persona (HP) for PCs, Mundane Persona (MP), Evil Persona (EP), Heroic Personage (HPg), or Other Persona (OP) for NPCs), skills are K/S Areas and skill ratings are STEEP, magic is Magick, spells are Castings, spell-points/manna is Heka, luck points are Joss Factors, money is accounted in BUCs, combat rounds are either Action Turns (5 minutes), Battle Turns (30 seconds), or Critical Turns (3 seconds), and on and on and on. This makes reading the rules a chore, as you constantly have to flip back and forth to the glossary, and even for someone who's just playing the game makes it seem more alien than it is (since almost nothing on the character sheet has a familiar name).
Mythus is pretty emphatically not a game for everyone -- it takes the trends of late-Gygax-period AD&D (i.e. Unearthed Arcana, the Gord books) and amps them up to 11, so that every character feels something like an AD&D cavalier or barbarian, with a long list of detailed talents and special abilities and a well-defined role within the setting right out of the gate. Anyone who finds the rules of AD&D too complex or the specific flavor and details too constraining will feel the same way but more more about Mythus. But for those who really dig the baroque High Gygaxian flavor -- who love stuff like the discourse on magic circles in S4, or all the lists of random crap in the DMG (reputed magical properties of herbs and gems, types of insanity, the complex sub-systems for hiring henchmen and generating the characteristics of sages, etc.) Mythus is a treasure-trove of stuff like that. Even if you don't want to play it as itself and just want to mine it for ideas for AD&D you'll be able to reverse-engineer probably a dozen or more new classes, hundreds of new spells, and various other fun and flavorful additions (several of which I already submitted to the "Dangerous Dungeons" board -- like the detailed Birth Order table, Joss Factors, and the knacks & quirks tables).
I really think that Mythus represents the true evolutionary next step from AD&D -- AAD&D, if you will -- and that, just like every OD&D fan should at least own and have read the AD&D books even if you choose not to actually play that game, every AD&D fan should own and have read the first two Mythus books. But then, you knew I was going to say that
