Hat tip to The Swivet for pointing out the discussion of Unlikeable Characters over on BookEnds LLC.
The comments are quite interesting, mentioning the "Save the Cat" strategy and, of course, the TV character House.
I'd add the personal fact that, given a truly unlikeable narrator or MC, I'm likely to stop reading even if I'm interested in the plot or mystery or whatever. Cory Doctorow's award-winning Down and Out in the Magic Kingdom - which you can download for free here - lost me because the main character was annoying, insane and stupid, even though the milieu and the mystery were fascinating.
Neil Gaiman's Anansi Boys nearly lost me for similar reasons.
For me, I won't spend six hours with someone unless I'd enjoy, um, spending six hours with them. Life's too short to read bad books.
On the other hand, there are lots of books I've read and loved, that I currently can't stand. Mark Twain comes to mind. It's entirely possible that someday I'll come back and read Magic Kingdom and absolutely love it. And in any case, I'd happily pay full price to see the movie.