I can't go along with this being one Beckett's more difficult novels, or for completists only, and not just because it's my personal all-time fave rave. From a biographical point of view, Watt marks the point where Beckett pretty much threw off the influence of Joyce, but before he self-consciously turned himself into the anti-Joyce. This brief state of affairs resulted in a fantastic, hilarious book that has everything - semi-vigorous ambulating, crack-up dialogue and rock-throwing action! This is Beckett's funniest work, and also contains some of his best discriptions and most memorable speeches (particularly Arsene's monologue), and is one of the easiest to read (allowing that Dream/More Pricks and Murphy are a tad insufferable, and thus a bit of a slog).
The impression that Wattis difficult may stem from the idea that there is some enlightenment within the text that the hapless reader is obliged to decode, deconstruct or otherwise deduce, but the book is more likely a dramatization, and an inflicting, of confusion. If this is found acceptable, the book is an intense pleasure to read and just maybe exceeds the Three Novels in this aspect.