There is a second Atari 800 emulator project running concurrently to the Joysoft epic. This is the Emu XL , from Draco of YesCrew. Emu XL is a much more optimised effort, but much less complete. The current version stands at v0.43, with a limited selection of screen modes, and a lot still to do generally. It is a lot quicker than Atari 800 though, coming in at over full speed on a Centurbo 2 machine.
Software compatibility is still quite limited.
I spoke to the maker of this software, Draco, at the Easter Error in Line part 2 party. He advised that development had been frozen for the time being, but was very likely to unfreeze if or when any of the heavy-duty Falcon accelerators such as CT60 or the Tempest came out. In his opinion, the current Emu XL was as far as it could go in a standard 'slow' 16mhz Falcon. Another famous old 8-bit platform was successfully emulated, as Christian Gandler managed to get the ZX Spectrum, childhood favourite of many of today's top coders, onto the Atari. This is quite old, as emulators go.