Here is what Nabokov had to say on his views of Finnegans Wake in comparison to Ulysses:

You have granted that Pierre Delalande influenced you, and I would readily admit that influence-mongering can be reductive and deeply offensive if it tries to deny a writer’s originality. But in the instance of yourself and Joyce, it seems to me that you’ve consciously profited from Joyce’s example without imitating him-- that you’ve realized the implications in Ulysses without having had recourse to obviously “Joycean” devices (stream-of-consciousness, the “callage” effects created out of the vast flotsam and jetsam of everyday life). Would you comment on what Joyce ! has meant to you as a writer, his importance in regard to his liberation and expansion of the novel form?

My first real contact with Ulysses, after a leering glimpse in the early twenties, was in the thirties at a time when I was definitely formed as a writer and immune to any literary influence. I studied Ulysses seriously only much later, m the fifties, when preparing my Cornell courses. That was the best part of the education I received at Cornell. Ulysses towers over the rest of Joyce’s writings, and in comparison to its noble originality and unique lucidity of thought and style the unfortunate Finnegans Wake is nothing but a formless and dull mass of phony folklore, a cold pudding of a book, a persistent snore in the next room, most aggravating to the insomniac! I am. Moreover, I always detested regional literature full of quaint old-timers and imitated pronunciation. Finnegans Wake’s facade disguises a very conventional and drab tenement house, and only the infrequent snatches of heavenly intonations redeem it from utter insipidity. I know I am going to be excommunicated for this pronouncement.

Ulysses towers over the rest of Joyce’s writings, and in comparison to its noble originality and unique lucidity of thought and style the unfortunate Finnegans Wake is nothing but a formless and dull mass of phony folklore, a cold pudding of a book, a persistent snore in the next room, most aggravating to the insomniac! I am. Moreover, I always detested regional literature full of quaint old-timers and imitated pronunciation. Finnegans Wake’s facade disguises a very conventional and drab tenement house, and only the infrequent snatches of heavenly intonations redeem it from utter insipidity. I know I am going to be excommunicated for this pronouncement.

Source: http://www.kulichki.com/moshkow/NABOKOW/Inter06.txt

EDIT: Garbage formatting on reddit took me an age to make it not double-spaced.