Everybody always talks about Nabokov's prose (and it is beautiful), but for me, what's so impressive is the shear power of his imagination. Nabokov's description is so detailed that you're left thinking he must be able to hold a nearly photographic image of the space in his head.
It had suddenly begun to snow, though the sky was pure platinum, and the slow scintillant downcome got reflected in the silent looking glass.
An ordinary writer would simply describe the snow, but Nabokov's mind goes straight to image of the snow in the mirror.
In a very small dining room, where Pnin contemplated arranging a buffet supper for his guests, a pair of crystal candlesticks with pendants was responsible in the early mornings for iridescent reflections, which glowed charmingly on the sideboard and reminded my sentimental friend of the stained-glass casements that colored the sunlight orange and green and violet on the verandas of Russian country houses.
This sentence (it's only one sentence!) shows Nabokov's imagination to its fullest extent. He starts with an object (the candestick) then comes up with a good, unfamiliar description (the iridescence). Most writers would stop here, but our boy keeps going. His prose leaps gracefully to a remembered image: the stained-glass window's of Pnin's childhood. And Nabokov does this kind of thing all the time; it's on almost every page. (The description of the world seen through young art prodigy Victor's perspective is particularly impressive.)
On the whole, though, Pnin is---well, it's okay. The prose and setting description is incredible, but the plot meanders. Nabokov produces interesting characters and then fails (or refuses) to have them do anything particularly interesting with our main character. The chapter ends right as Victor meets Pnin; when the next chapter begins, Pnin is driving and Victor is off somewhere else, never to return. Liza appears once and never recurs. Nabokov keeps serving up page after page of beautiful sentences, describing ultimately meaningless character interactions. The reveal at the end disappointed me; it seemed metafictional rather than plot-appropriate. Not a bad book by any means, but a little boring sometimes in spite of its genius.