I just used simple green a week and a half ago for the first time. It did absolute Wonders on my metal models after an overnight soak- I couldn't have been happier!
On the plastics (a hammerhead and a few tau gun drones) it took a LOT of scrubbing to get most of the paint off, and I couldn't get it out of the crevices. I think you'll have to soak it several times, or for a much longer time, to get at the plastics.
Fortunately it doesn't harm the plastic at all, so you can keep 'em in there as long as you want. The fumes are terrible, though, so you should put it in a container that you can put a lid on!! I've heard the lemon-scented version smells less, but I couldn't find it.
I got it at Home Depot
Others might post with different solutions to try for plastic... the simple green won't hurt it, but it'll be a mess, and it'd be better if you could do it in one go. So I'd search the forum and see about alternate products. I traded for some Tau pirahnas that were stripped in brake fluid, and they came out perfect. However, it is toxic and you'll have to dispose of it in the correct manner afterwards (whereas simple green is nontoxic, but doesn't do as good of a job on plastics).
Hope this helps