I was lending a hand to some friends when they were doing their own house remodel, and I cut all of the round holes for the light canisters in the ceiling and the rectangular holes for speakers in the walls by using a Dremel rotary tool with the cutting guide attachment and the drywall bit. Makes short work of the job, and gives a much cleaner cut than a drywall saw.
https://www.youtube.com/watch?v=tSJR5dDOo5U
Oh, and I do not in any way work for Dremel, but if they want to throw me a bone - hey, I'm not proud! I could use some extra bucks to get that 65" LG OLED UHD TV!!!