I used castor oil and didn't have any problems. I had a malpresenting baby and 3 days of active labor for 8 hours and then nothing, active labor, then nothing
My midwife suggested it to her apprentice over the phone without checking the position of the baby and the real reason I was crapping out.
Anyway, I didn't have a problem with the castor oil, in fact, I credit the recent crapping to my lack of poop as dd descended.
My understanding of the way castor oil works is this- the molecules are too big for your body to know what to do with. You don't really digest it or absorb it, because your body is so freaked out by this thing it can't process that it forces you to evacuate it very quickly. This means that because it doesn't get into your blood stream, it doesn't cross the placenta. It's the stimulation of the lower digestive track that urges labor to progress forward. Which means that an enema can do something similar. The reason I think some babies pass meconium with CO is because starting labor without waiting for baby to let the first hormones fly creates a "hostile" environment for them and starts them out under stress. This is the reason I believe that one of the midwives in the practice I used (J, for those of you who know my midwives and their issues) is so freaked out about past due babies and the way they "tolerate" labor. I don't think it's the past-dueness of the baby that causes the problems, but the stressful environment created when they are pushed to be born.
I think in the right circumstances castor oil can be very helpful, but I consider those circumstances rare. I certainly didn't need castor oil, I needed a big hard baby head on my cervix!