They're quieter than with nothing. Im sure that if one were to murder someone across the house or next door etc; one would be hard pressed to jump out of bed to see what's going on.
But many a person has been disappointed when they get to play with them personally.
Yes, it's a suppressor, on a .45 it just changes the sound to a point that you don't hear a gun, you here a loud noise.
A .22LR with a can makes a snappy click, if you've ever heard a .22 short with a can, it literally is dead silent. So silent that you only hear the firing pin spring. Me and my brother use to shoot cans in the backyard at 3am and the loudest thing was the cans getting punctured.