If youre actually using them for something besides just blasting, they have a definite use. I use one on one of my AR's for garden and yard pest duty, and it works great. I can shoot off the floor of the car port shooting 5.56 out of 10" or 16" AR, with no hearing protection and I can actually hear afterward. If I shoot a 22lr in the same spot the same way, I cant hear for a couple of days.
While they arent "silent", they do remove about 95% of the muzzle blast. With supersonic rounds, you still hear the round breaking the sound barrier as it goes down range, and there is that crack. And I swear, the closer you are to the target, the less you notice that. To me, the longer the round goes, the more noticeable it is.
As far as accuracy goes, I haven't seen any real change either way. With some guns, there is a definite POA/POI shift, but not with every gun. Id say about half of my AR's see it and the other half don't. And when it happens, its usually no more than a 1-2MOA shift, and in random directions.
Ive never noticed any recoil reduction with one mounted. Maybe the added weight gives that impression.
If Im planning on shooting more than a couple of rounds at an outing, I still wear hearing protection. I can shoot 50+ rounds of 5.56 without it, but later on, my ears are "dull" and still ache like they do without the can on the gun. The difference is, I can still hear. One round of unsurpressed 5.56 and Id be deaf for a week. The "hearing safe" thing is a bit of a misnomer.
And a suppressor or not on a pistol inside, is still going to be loud. Better than not, but still.
The NFA aspect of all this is a PITA and total BS, but it is what it is. Maybe ask the NRA for some help and relief.
If they were available over the counter, a lot more people would be using them. Other than the PITA involved these days, dealing with the NFA isn't a big deal either, and its a "tax" not a permit. Although it does have the feeling that it is.