More accurate than FCEUX, less accurate than Mesen. I do cut a few corners to save processing time which doesn't seem to affect any of the games. Even then, I hold FCEUX and Mesen in very high regards.
My personal opinion, it's about convenience:
Mesen really is a great emulator, and if you are using it, puNES, fceux, or any other emulator for years and like it - you should stick with it.
Of course on the other hand FBA/N fans can now play NES/FDS games in their favorite emulator with their favorite UI.
best regards,
- dink