While some people swear by simple green, I had never heard of it being used for paint removal until recently.
However, I can tell you Castrol Super Clean(CSC), available from local auto parts houses, walmart and other places works great for me and a lot of others. It seems to take the least amount of time to get paint off (Of course the longer the paint has been on the plastic, the longer it might have to sit). It also seems to be one of the most economical solutions I've found. I pay $3.95 (local Auto Parts store) for a gallon of the stuff and it lasts along time if kept in the original container. It can/will evaporate if left exposed to air. CSC comes in a large purple jug, kinda like a bottle of anti-freeze.
Here's how I use CSC...
I fill a Sterilite Container(You can use RubberMaid,Tupperware,Gladware, etc..) (Mine's about the size of a shoebox) that has a lid about 1/2 full with CSC, then I just place the parts in it and let them soak. Generally about 15 minutes to 30 minutes for recently painted pieces (painted within the last week or 2). I then take the parts out and give them a light scrubbing with a toothbrush while running water in the tap(This is to rinse away the old paint, as well as the CSC). Once I have the paint removed I then thouroghly rinse the part in water. I then let the part to thouroghly dry before re-painting. The longest I have ever had to leave parts in CSC was for 3 days while stripping a model which had paint on it for over 3 yrs.
I hope this is off some help! Good luck and let us know what you use and how it turned out.