It's rare that I give up on a book completely. There have been a few.
When I was a teen, I gave up on "God Emperor of Dune" after thoroughly enjoying the first books in the series. I re-read the entire series as an adult, maybe ten years ago and found it much more readable. I think the deeper themes went right over my head as a teen and so the much less action oriented (on the surface) nature of "God Emperor" was too dull.
A recent one that I really wish I had given up on was Todd McAffrey's "Dragon Harper". That was just brutal: childish plot, thin characters, and a writing style I couldn't stomach. I could go on and on about how bad that book is. I don't think I'm ever likely to pick up another Todd McAffrey effort. It's a shame (to me) because I so loved Anne's Dragonriders and Harper Hall books when I was young.
The thing that this thread highlights for me is how different people's tastes are. I really enjoyed a lot of the books being dismissed here (Dark Tower, Lord of the Rings, Pillars of the Earth, etc.)
Another one that I put down and have never gone back to (but I still aspire to reading one day) is "Gödel, Escher, Bach" by Douglas Hofstadter. I felt like I was incapable of understanding what he was saying, but that I really should be able to get there if I thought deeply enough.
