I considered trying castor oil at some point, but had to pass as I don't fancy spending the better part of a day on the loo. The good thing about suppositories is that you have a rather small time frame, with bisacodyl I know it will take anything between 20mins and 1 hour, maybe less with 2 suppositories. I can plan around that and although there will be some after effects in the hours following BM I know I will be in control.
With castor oil (or any other laxative taken orally) it's a different story entirely. You can't be sure that the laxative is going to take effect in the next hour, or the next 2 hours etc.
Take Dulcolax pills, for example. On the box it says that it will take anything from 8 to 12 hours. At one time I took 2 pills around noon and thought I'd be safe till evening at the very least, well I was in for a surprise when a sudden strong urge set in around 2pm, I was lucky to be at home at that time. A couple of weeks later I learned on these forums that these pills have a special coating. I knew that, it says so on the box. What it didn't say is, that anything alkaline will remove that coating and set the bisacodyl free immediately. I said to myself, here we go, that must have been it. So I started another experiment, I took 2 pills and followed up with 300ml milk to see what would happen. Would it take 2 hours or rather 8-12 hours. Well it was closer to 8 hours and that was only because I got impatient and forced a BM with my white, torpedo-shaped friends (the urge from the pills was there alright, it was just too easy to resist at that point).
The bottom line is, oral laxatives are unpredictable and I steer clear of them most of the time, unless I know nothing's going to come up for the next 12 hours or so.