I finished this book. A friend dared me too, so my literary pride was at stake. It helped that I was on a journey by air halfway around the world when I was halfway through. I was slowing down, but this gave me enough time with nothing better to do but finish the bloody thing.
When I got to the description of a lightbulb that took up several pages I became sure that Thomas Pynchon was just trying to fill up space. I'm sure that as descriptions of lightbulbs go, it's a shining light. But it does not serve to advance the story in any way. Story? There may be a story in here somewhere struggling to get out. This is literature, we don't need no steenkin' plot.
As ErisDiscordia says, Gravity's Rainbow is like an LSD trip: it start of mostly normal, gets wilder and crazier, heading off in all directions, constantly novel, but after five or six hours of this you just feel tired of the constant distractions and start to wonder when it is going to end. And then it peters out inconclusively.