I find it much more satisfying to emulate the very lowest levels of hardware/software. Then the fact that very complex software, such as an unaltered app, runs

I find it much more satisfying to emulate the very lowest levels of hardware/software. Then the fact that very complex software, such as an unaltered app, runs as it was intended to, on top of your emulator is verification that your emulation is indeed correct. Emulating an app is trivial compared with emulating the underlying structures. Actually, emulating an app is just doing a 'clean-room' rewrite.