From my point of view as a book reviewer, I have not done this so far, but I can understand hating a book so much that you just can't bear to go on. For example, I remember reading Devourer of Book's review for a book so bad she had to give up. But she admits this in the very first line of her review. Regarding Ebert's movie review, on his blog, Ebert writes that his own editor told him "Your original review is clever and well-written but I think morally dishonest because you conceal your MO until the very end." Ebert's response is that the logical flow of the review didn't require him to admit that he hadn't seen the movie until the end.
I'm interested in your take on this. As a reviewer (of books or movies), are we obligated to finish the whole thing? If not, is it valid to write a review of something we haven't finished?
Update: Thanks to Ali for pointing out that Ebert went back, watched the whole movie and wrote a new review. Definitely the right thing to do.
As an aside, here's a line that popped out for me, as a book reviewer, from his blog post about writing a new review:
In even my negative reviews, I try to give some sense of why you might want to see a film even if I didn't admire it.