We used it to augment labor, meaning that I had been having contractions for a long time and was tired and had an empty stomach. It was hell on earth for me, and I would not do it again under any circumstances. I've heard that people that try to induce with it usually don't have the problems that I had, simply because their bodies are not already in a compromised state. But it also probably just depends on individual tolerance.
Aside from the misery, though, it can get through the placenta and affect the baby. It can give baby the runs just as much as mama. So then you're dealing with meconium and possibly distress. IMO not worth it. I have to agree with mehndi mama too, any means of inductions, whether "natural" or not, will interfere to some degree with the natural process. And personally I would rather put my trust in the natural process than in castor oil.