Pnin as a novel is haunted by what it could be. There parts of this book which are simply brilliant (end of Ch. 5 in particular), but they are islands amongst the merely mediocre. That is not to say the book is bad, anything but in fact, I just seem to be holding it to higher standard on account of it being a Nabokov.
Pnin as a character is one of the most likeable men in fiction, I dare you not to get completely taken in by his clumsiness, awkwardness and eccentricities. Evidently intelligent but marred by his complete lack of social skill, Nabokov creates all manner of hilarious situations for Pnin to navigate, simple non-issues are converted by Pnin into chaos and of course the most complex of problems are handled efficiently with little alarm. Pnin's turbulent backstory is revealed slowly and in some cases, darkly, but always ends up endearing Pnin further to the reader.
The prose is nowhere near as fantastical as Nabokov's other work but infrequently there is the little explosion of alliteration and assonance. Pnin's particular dialect is handled extremely well and never short on comedy. The odd flirtation with French is not taken to the extremes as with H.H's fancy prose but seems only for the embellishment of more comedy.
In a similar vein there is little narrative experimentation, you won't have to sit there with a notebook trying to unravel meta-narratives but you will have to contend with a particularly unreliable narrator. The narrator's voice imposes sporadically to repeatedly express disappointment, disdain and occasionally, disgust, with Pnin. All just to make you doubt the level of bias in Pnin's presentation every time the narrator intervenes. Then again it wouldn't be Nabokov without a little head-scratcher.
A solid, funny (in some cases belly-achingly so) and enjoyable novel the only flaw for me is in the last chapter - I can see why its necessary but it just seemed to go too far past the end. Its worth reading just to meet and get to know Pnin.