I used a Roto-zip with the circle cutter attachment the last time I had to cut ceiling tiles. We set them on top of a garbage can, really keeps the job neat. I bought one of the Ideal adjustable hole saws, it's sucks, big waste of money. Greenlee does make a non-adjustable hole saw if you have a lot of tiles to cut. Someone told me (in this forum) to run the drill in reverse for better control.
