For about four years up until several months ago, I was bleaching my own medium brown hair blonde. I stopped doing it myself because I had a scary episode in August which left me with extremely damaged, spongy-textured hair. I realized that a similar mistake in future could very well leave me bald (I can practically hear all the hairdressers reading this post saying, "no s***"), and now my hairdresser is now doing the bleaching for me.
Anyways, after my scary episode I was ready to throw money into my hair to get it back to some semblance of health, allowing me to keep as much of it as possible and allowing me to continue with (professionally) blonding my hair. This is one of the products that seemed to come highly recommended. I've been using it about once a week for about 3 months, leaving it in for the recommended 5 minutes each time. I use it after shampooing, before my usual conditioner. I use quite a bit of it to ensure all my hair is saturated. By the way this stuff smells strongly of bananas which I'd have thought I wouldn't like, but it's actually quite nice.
This product does make damaged hair much more manageable and feel healthier. In the short term, after I've used it I notice it's tamer, softer, and less frizzy or prone to flyaways. I'd use it with every shower if I could, but 1. It would be annoying to wait 5 minutes in the shower each time and 2. I've read that it's not a great idea to overdo protein, and anecdotally I've noticed counteractive effects from using another good protein product, Beyond the Zone Protein Cocktail spray, twice in a week: more dry and frizzy hair. Btw I prefer K-Pak Reconstructor to the Beyond the Zone stuff, in case you were wondering.
In terms of long term effects, the last time I went to my hairdresser about a week ago, she commented on how much healthier my hair feels and asked what I'd been using. I did 2 Olaplex treatments shortly after my hair disaster in August, but I've seen my hairdresser a few times since then, and since then the only difference has been the K-Pak Reconstructor. I've read time and again that it isn't scientifically possible to undo damage to your hair, but fwiw my hair absolutely has improved since I did all that damage 4 months ago. Some of it has probably broken off, which would make my hair seem healthier on the whole, but definitely not all of it (a lot of my hair was damaged).
Anyways, will repurchase.