I read Gravitiy's Rainbow in an intense four-day sitting during college. Loved it. Pynchon's prose seemed quick and easy at the time, about 20 years later I tried it again and only got halfway through it. Proust, him I had problems with, but as wsieber said, it may have been the translation. Stephen King's prose was like daggers in the eyesockets and toothpicks under the fingernails: ooff. I remember Mervyn Peeke's Gormengast being entirely unreadable.