I found a good article on coffered ceilings, from Finehomebuilding. We want to add them to our living room. The ceiling was dropped a few inches in the 60's, so we'd have to remove the ceiling. I'm sure the plaster underneath would have to be removed as well. The ceilings are 8'-4" currently on other parts of the main floor. The dropped ceiling its 8'-1". We were thinking the coffered ceiling would be 5.5" thick, so I guess the ceiling would be 7'-10.5" so hopefully it wouldn't feel too low.
Here's a reflected ceiling plan showing a layout we're thinking about. We're thinking of just doing crown around the fireplace. We'll keep the crown and other mouldings simple, since all the other mouldings in the house are simple. We were going to start on this project now, since I have a week off, but I think we're going to try and wrap up some projects first.