I used to swear by Simple Green, but the last few times I've used it, it hasn't done much for me. I think it has a lot to do with what kind of paint was applied, and in many cases, I don't know, as I've bought the units second-hand.
I had a bunch of 40K plastic Eldar that I soaked in Simple Green for months and then scrubbed with a toothbrush and it barely lifted any of the paint from the surface. I think I tossed those things back in the Simple Green and they are probably still in there to this day. I should likely go find them and see if they are just goo now.
Most recently, I soaked some 'Mechs I got off of eBay. Even after a week in full strength Simple Green, I've gotten next to nowhere with them, except to make the paint very gummy and even harder to remove. They are sitting in Acetone right now to better effect. I think I'm going to stick with Acetone for anything metal going forward.