Nabokov's writing can make me grin in the same way as when I watch Lionel Messi or Johan Cruijf play football - the exhiliration of seeing a master in action, in complete control of his medium.
Pnin is endearing and lovable, while at the same time being consistently cringeworthy or absurd. He exploits are painted vividly on a meticulously realised backdrop of Nabokovian Americana. Familiar settings like universities and diners are embued with fresh life - descriptions I may have been tempted to skim in another book bear repeated re-readings.
Beauty is to be expected from Nabokov, but the strength of the humour may surprise you. The physical imagery of Pnin, with his strange, top heavy body and bald head combines with verbal humour ( "I never go in a hat even in winter") very effectively.
An undercurrent to the humour is that Pnin is frequently at the wrong end of it - the reader snickers at some gaffe poor Pnin has made, but in the next passage frowns at other characters laughing at him too.
It's short, but its images and scenes will leave a stronger imprint on your memory than most longer novels. I can definitely see myself rereading this in years to come.