While I did read this book because of its presence in the Appendix N, I was more interested in having a fun read than looking for clues about the origins of D&D. I found the references regardless of this - they are very obvious, and anyone trying to understand where some ideas from D&D came from should read this book.
The Eyes of the Overworld tell a story of Cugel the Clever, a roguish anti-hero that must travel back home after being forcibly transported to distant, unknown lands by a rival magician.
Did I enjoy it? Well, yes and no.
The book is definitely well-written and full of ideas for any D&D game. The author is amazingly clever in both creativity and styles. Each chapter reads like an adventure, and Cugel always seem to find a clever, if often immoral, way through his problems.
Vance's style, it seems, must have been a huge influence on Gygax prose; it is certainly sesquipedalian, a word I learned form Appendix N reading...
And the book is definitely funny at times. It never takes itself too seriously - but is is not entirely comedy or satire as well. The Eyes of the Overworld, and its protagonist, Cugel, are firmly entrenched in the picaresque tradition. Cugel is arrogant, selfish, often smart but always unwise, and never seems to learn anything from his misadventures (at least in this book, and the ending might be a symbol of that) - but he is not quite the villain, since many of the characters in the story are equal or worse than him. There are not even hints of damnation, redemption, tragedy, etc. - Cugel is what he is and doesn't seem to care for changing.
BTW, he is also an interesting character in D&D terms - chaotic to the core and with abilities that would fit thieves, wizards and bards, he seems to be the prototypical "rogue", maybe even more than the Gray Mouser.
His "character sheet" is interesting and worth quoting:
Cugel was a man of many capabilities, with a disposition at once flexible and pertinacious. He was long of leg, deft of hand, light of finger, soft of tongue. His hair was the blackest of black fur, growing low down his forehead, coving sharply back above his eyebrows. His darting eye, long inquisitive nose and droll mouth gave his somewhat lean and bony face an expression of vivacity, candor, and affability. He had known many vicissitudes, gaining therefrom a suppleness, a fine discretion, a mastery of both bravado and stealth.
If you're okay with that charachter, you will probably like the book.
For me, the despicable nature of Cugel, while amusing, prevented me from caring for his fate at times - nor did I hope for his downfall, since there was no one to root for and most of the world's population seemed unconcerned about the imminent destruction of the Dying Earth (I should note I haven't read The Dying Earth in its entirety, only a couple of short stories).
I don't mind the break from powerful heroes of the Appendix N such as Conan and Elric, and I certainly enjoy Fafhrd and the Gray Mouser with all their shortcomings, but Cugel at times seems to be too selfish and unwise even for an anti-hero. Jurgen, the picaresque hero from "Jurgen, A Comedy of Justice" might be a better comparison (and Jurgen was a bit funnier, although not as useful for D&D).
But the book, and even Cugel, certainly has its moments - some of them unexpected and amazing, such as the discussion about the origins of the universe (Cugel, as usual, doesn't seem to care).
In short, a great book, with funny moments and numerous ideas for your D&D adventures, but do not go in there looking for an heroic protagonist!