I actually saw The Imposter at an indie theater here. It was good, but I have to say that it didn't quite live up to the trailer for me. But you're right, it is fascinating. You have to wonder how those people, even for a second, believed that this was their kid. And you also have to wonder what really happened to the real one.
I believe I started Into the Abyss once. Not sure I finished it. In either case, it didn't leave much of an impression.