Well, I've only read the first Dying Earth book by Vance so far, which is a collection, not a novel, by the way. Early Vance was more quantity than quality, so I guess his reputation rests on later works (later than Big Planet at least). He wrote the stories while still in the navy in the mid '40s, but couldn't sell them to magazines, and it was not a hit when published in book form by a small and indifferent publisher in 1950. Young Robert Silverberg was apparantly crazy about it, though.
The shorter stories of the collection are good, and one can see the first traces of the D&D magic system, but the two longer stories are more routine pulp action with the second story following the same formula as the first one. Overall a bit over average and weaker than CAS' stories IMO. I'm looking forward to reading the rest of his Dying Earth stories.