For anyone considering the implant for a rooster, I am sharing my experience. I had one placed in a Polish roo when I noticed his crest growing in with the typical, spiky, rooster look rather than the soft, rounded shape on a hen. He was not yet crowing or showing any male behaviors. From what I had read, the timing was perfect. Between the implant and the appointment, I spent $220. His feathers continued to grow in as would be expected for a rooster. It is now one month later and he is starting to crow. I would have to call this a complete failure at this point. From the other posts, it must work sometimes but know that it isn't a sure thing even if implanted before they are old enough to crow. If you have fallen in love with a rooster you can't keep, and are willing to gamble a couple hundred dollars, you could give it a try, but I won't be doing it again.
I have also used this implant in hens with laying issues and it has been a literal life saver. One had such a severe infection from egg yolk peritonitis that my vet said he has never seen a bird recover after being so ill. Once we got her through it, I had the implant injected because she was at risk for a recurrence. She molted almost immediately and her comb shrunk. That was 6 months ago, she is still not laying and she is strikingly beautiful. She appears to be glowing with health and in perfect feather. I had another hen laying paper thin eggs and no amount of calcium supplementation made a difference. They were so thin that they broke in the nest underneath her. That left her constantly covered in eggy goo, and me with a messy nest to clean up almost daily. I don't know how she was managing to lay them since the shell wasn't hard enough for her body to push against. I didn't want to risk peritonitis with her, and I was tired of the mess, so I had the suprelorin implant put in her as well. She followed the same pattern of molting and comb shrinking as my other hen and now looks fantastic. I'm so happy to not worry about her and to not be constantly cleaning her favorite nest.
In short, my experience has been the implant is a life saver for hens and a big disappointment for roosters.