From what I read of the early caloric restriction studies on animals, it tended to greatly increase the healthy adult portion of the lifespan rather than adding a lot of geriatric time. That was the great hope for the human version too, but all the accounts I've read do indeed sound miserable.

I'm curious about fasting though. It's very easy for me to believe that our bodies are designed to go without eating for a while, much like we need to sleep. In nature we wouldn't really have needed any prodding to miss some meals so there wouldn't necessarily be a corresponding drive like thirst or sleepiness. I haven't tried it but personally I've never suffered much from missing a meal or two... seems like it wouldn't necessarily hurt quality of life if you were used to it.