I'm still worming my way through it, but as a huge fan of the Lyonesse books (and Vance in general) and the BRP family of games, I can safely say that so far, this is the most impressed I've been with a product since I picked up Elric! The layout is superb, the prose is great, the magic systems seem inspired, and (this is the big one for me) I can't think of anything I would house-rule or change as I read it.
There have only been a handful of other games that I had the same initial reaction to (Elric!, and Dungeon Crawl Classics perhaps?). It's everything that's great about Mythras, but tuned to perfection to capture what makes the Lyonesse books so special.
I can't wait to start my next campaign here.
[5 of 5 Stars!]