Yeh, I used online play in... it was before BSNES, so it must have been SNES9X, although I seem to remember having some sort of exchange with Richard over it so I'm kinda confused. Pretty sluggish, even over decent broadband, but that's just what comes with the territory when you need to replace an inch or three of copper trace or a few mm of silicon with miles and miles of fiber...
To refresh your memory, the discussion with RB was about putting it into one of his NES emulators, as it was already implemented in SNES9x IIRC. At the time, he was against adding net play to his emulators or to emulator enhancer.
To answer the question, both SNES9x and ZSNES for OS X support network play. BSNES does not, AFAIK.