with the f8500. it appears as though auto changes based on room conditions. if I turn the lights on, the MLL rises to match off/bright room levels. if I leave the lights off, the MLL remains low like the dark room setting.
setting it to bright room, increases the peak luminance, but only if you have your contrast high. at a setting of 90, it basically makes no difference. at 100, it was almost 10ftl brighter iirc.
based on my experience with my f8500. I'd recommend the following
off: only if you are very sensitive to the changing black level and it's distracting you
bright room: if you almost always watch in a bright room, have the contrast maxed out, and want as bright of image as possible
dark room: if you almost always watch in low ambient light and want the lowest possible MLL
auto: if you often switch between bright and dark ambient lighting, and want the highest on/off contrast ratio
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One