Bondo takes some practice to master, and initially, you're going to think it is perfect for all applications. Some more practice and you'll see that it's great for filling in cracks, and that's it. It's a filler. If you try to make a whole piece covered in bondo, it's gonna crack when you move it and it's gonna take longer to sand it smooth.
I like to get Sintra at Artist and Craftsman (it's cheaper in store than on their website) and use the biggest parts of a pepakura model as a template. Then I try to fill in with Bondo and almost carve out the smaller details with a sanding drum on the Dremel. It's not good for perfect replicas, but it's a good start for a base to build on (for example, a simple helmet that can become more detailed or my first X-Wing Pilot Chestbox).