As a standalone book, Lurulu isn't a great success. It's short, and it depends heavily on what went before. (Though there is a nice summary of Ports of Call, which is well worth reading, as it describes that book in even more colorful terms than the book itself.)
As a sequel, or considered as the final part of a single volume inexplicably split by some editor into two parts, it's quite good. It continues the story, wrapping up a number of loose (and even new) threads in a satisfying manner, all with the characteristic Vance flair.
But it's as the coda to an astounding writing career that the story (both books taken together) really satisfies. It's not Vance's strongest plot, and it doesn't have his weirdest settings, or his most inventive machinations. But it does encapsulate nicely a lot of the things I love about Vance.
The story is an unabashed travelogue in the finest Vance tradition. It showcases his never-ending, inventive flow of the weird and wonderful, of bizarre natural beauty and equally bizarre human venality. It describes customs irrational and cruel, habits set in stone, all seen from the eyes of one of Vance's most sympathetic protagonists, and an unusually large and well developed supporting cast. Emotions simmer and stir without ever overflowing. There are philosophical speculations galore, deceptively deep but cast as casual considerations and throwaway curiosities.
If the concept of 'lurulu', introduced at the end of Ports of Call is never fully exploited, its very ineffable nature as a vague sort of yearning is a fitting cap to a career of genius cataloging and trying to satisfy exactly that need. Whatever 'lurulu' is,