I recently had a PES moderator fitted to my 6.6x55 and think that it's made a really good difference. Reduced noice, muzzle flip and the deer, if standing in a group, do seem less disturbed, so more chance of a second shot.
The only negative is the weight and increased length. The increased length is a problem in my enclosed highseats.
Steve Bowers at Specialist Rifle Services fitted mine - seems to have done a great job and clearly knows more about rifles than is good for a man!