I know that typically we use 5/8" drywall for the ceiling (which is what I have always done) due to issues with sagging, but how big a difference does this really make? I will be finishing a basement ceiling where the joists are 16" on center...do I really need to use 5/8" here? 1/2" certainly is cheaper and much easier to work with, but I definately don't want to wish I had gone with 5/8" later on down the road.
I have used 1/2" drywall on ceilings with no sag. In fact, it was 4' X 12' -1/2" sheets. I always assured that I fastened it properly with at least 1-1/4" coarse thread screws at 6" to 8" spacing. I did this the first time 26 years ago and the ceiling is still hanging to this day.