I don't think they're a good idea- i am totally against implating anything into an animals body that is not medically necessary- it's just not a good idea- and it could result in issues down the road.
As a vet tech- we spay and neuter countless animals every week- all of the male animals i've ever assisted with have done incredably well after being neutered- and no they do not "miss" their testicals
It's totally a mental thing for human men.
I've had more than one idiot come into the shelter and not want to adopt an animal because the all leave spayed/neutered- that's when i explaine to them why neutering is important, why it does not demasculanize the dog- it prevents testicular cancer, marking in most cases, roaming, and a plethera of other behaviorial and medical issues. Normally if you take the time to explaine to people why neutering is important they'll respond positively. It just take s a lot of patience working with idiots on a daily basis
I will definitely say Colin and all the other guys in my family ALL have their pets spayed/neutered and they all understand why it's important.
Another thing about neuticals is that you risk having the dog opened up a second time for surgery because if a tech or vet does isn't familiar with the implants- they may think they're feeling actual testicals when they check to see if the pet is fixed and if they feel those- they might result in attempting to neuter again only to find out their fake. So you put your pet through the anestesia and trauma of being cut open not only once but twice then- it's not a good idea. And most vets i know do not put a neuter tattoo over the incision site on the neuticals because it's usually women who get it done while their husbands don't know- so they don't want it to be a give away.