Other classics I hate the writing style of are 1984 and Children of Men. 1984 I made it to page 11... 6 times and fell asleep every time.
I found 1984 difficult at first, but the story picks up about halfway through. I liked it by the end, but it takes some work to get to that point.
There are plenty of books I've read that were painful to get through, but they were all required back when I was in High School (fuck you Scarlet Letter). I can't think of a book I read for fun that was just bad. Certainly, I didn't finish any and then managed to push them out of my mind.
The only thing I liked of Asimov's were his Norby Chronicles, but those are light and fluffy and not "serious" science fiction. They were pretty amusing, as I recall.
I read Norby when I was a kid. I don't remember how many stories I read, but they were definitely fun. I actually like the Foundation Trilogy. I think my history background helped. The newer books were disappointing, but I don't think they completely ruined by enjoyment. The last several chapters of Foundation and Earth came damn close, though. It also came close to retroactively ruining everything that came before, but, fortunately, I don't believe in retroactively ruining something.