My soffit is to hide mechanicals. It is hiding the HVAC ducting, main sewage pipe going out to septic, along with hot water pipes, propane pipes, etc. I also use the extra space to run much of my cable (RG6 from dish, CAT5e runs, etc). Going to be installing some insulation and then drywall over my soffit.
I'm not mirror imaging my soffit in the theater (or either room) for the sole fact I don't want to lose the headroom. Yes, the room will have somewhat of an unbalanced look, but I'd rather have the height back.
My soffit runs into both my theater room and rec room. I went with a sloped box soffit to help regain some headroom as I'm a tall fellow and my ceilings aren't huge.
Here are a few pictures of my setup:
Looking from theater room to rec room
http://www.30th-ta.org/hometheater/104_0401.jpg
Looking from rec room to theater room
http://www.30th-ta.org/hometheater/104_0403.jpg