For the .emu apps from Robert Broglia I'd say that they are defin. worth the buck as there is a big difference compared to many other emulators available for free. Most of them are based on freely available sources from emulators for x86 platforms, but I guess that it consumes a lot of time and dedicated work to port and finetune them for the android system. SNES9X EX from the same developer is free (I guess for copyright/IP reasons).
I like the fact that they all perform well and have a standardized user interface and think it's good to support a developer that invests into the further improvement of the apps. Another good example would be FPse - The Playstation Emulator.
Differences between paid and free emulator are sometimes minor, but this is the whole idea of the thread. To discuss and recommend each others favorites. This would allow a lot of users to gather all the information they need in one spot before trying a lot of emulators wasting precious play-time