Had my first experience with the .300 Whisper (essentially identical to the .300 AAC Blackout) a couple of weeks ago on a prairie dog removal trip with the good folks at Smith & Wesson. Using an M&P15 sporting an AAC can and shooting subsonic ammo from Hornady, the Whisper is indeed extremely quiet. All you hear is a pfffft when you press the trigger, followed by a thunk as the bullet impacts the backstop.
What the Whisper is not--at least with subsonic ammo--is tack-driver accurate. When aiming for targets as small as prairie dogs, accuracy is key. I found I could regularly dispatch the pests inside of 50 yards with the subsonic Whisper load, but outside of that range it seemed luck had more to do with rat survival than any degree of skill. Granted, my skill level isn't going to win any medals, but when I switched to a Performance Center M&P15 in .223 Rem. and used supersonic ammo, clearing the field of rodents out well beyond 100 yards was quite easy.
Another thing to consider is terminal performance. Because subsonic bullets are moving relatively slowly, they do not expand as quickly as supersonic rounds so designed. Therefore, while the bullets in the subsonic .300 Whisper were Hornady's 208-grain A-Max--a polymer-tipped match projectile that expands rapidly in supersonic loads--the low velocity caused them to behave more like FMJ rounds when impacting the bubonic plague vectors.
But ultimately, the .300 Whisper got the job done at appropriate ranges. On a larger target, effective range would no doubt be significantly extended. While not the ideal prairie dog round, it was easily the quietest rifle load I've ever shot.