After using it for 6 months. Well, first time I bought a professional shampoo (and conditioner of the same range) because I was extremely tired of waking up every day with greasy hair. Generally, with supermarket shampoos, I would wash my hair every 3 days (and I must admit, I would often let it go longer). I have very oily hair and dry ends, and of course, very fine hair (that's why I chose this shampoo), it is very long and I have never dyed it. I bought it because I saw many videos on YouTube by a professional hairstylist from America, not because I believed him and bought it, but because he explained exactly why my hair type needs this kind of shampoo (it's called blowout professor). Well, initially I have to say that the first time I used it, I was a bit scared. I have never felt my hair so clean (without being greasy, I realized that after drying it). There would always be something left with the supermarket shampoo, regardless of whether I used it 2 or 3 times. I am very happy with the product for the following reasons:
1) my hair stays clean for at least 3 days, which was not the case before, and I can go without washing it for a long time (from the second day it would be a mess and needed washing, but I wouldn't wash it because it's not good to wash your hair every day).
2) it requires a minimal amount to clean your hair. (always apply shampoo only to the scalp-roots, NEVER to the ends, especially with this shampoo, as professional shampoos clean very well, and that's why when we go to the hair salon, the hairdressers only apply the product to our roots).
3) my hair is very soft (even though I don't use a hair mask often).
4) it might provide volume, I'm not sure, but if you want volume, you should use some styling product. DO NOT buy it with the expectation that it will give you volume.

Negatives-my thoughts:
1) it is an expensive shampoo, BUT it doesn't require using a lot of shampoo like the ones from the supermarket, so it's worth it.
2) without oil on the ends and leave-in conditioner (not regular conditioner, leave-in is an extra step for healthy hair, it's a spray), my hair doesn't stay as hydrated as I would like, but I don't think it's the shampoo's fault.
3) the scent is not bad, it's okay, I would say it's indifferent, but if you use a scented leave-in conditioner after washing with it, for example, coconut scent, you won't have any problem, and your hair will smell strongly and nice.
4) I have noticed that when I wash my hair with this shampoo, more hair falls out. Of course, I'm not talking about a terrifying loss, I'm just not sure if it's the shampoo's fault or if it's because I wash my hair less frequently.
5) it requires styling with a blow dryer to really see the results, it's not meant to be left alone to dry.
6) if you don't use it correctly, you won't see results.

Summary:
I am happy with my purchase and would buy it again. However, I want to try other shampoos to have an objective opinion. Also, the shampoo finishes quickly compared to the conditioner (I use the shampoo twice, once doesn't do anything). Consider that for long hair, washing twice a week for 5 months at most, the shampoo is finished while the conditioner has only used half of the packaging.

Tips:
If you are thinking of buying it, you should consider that this shampoo is for fine and medium hair, NOT for thick hair. If you don't see much lather and foam with the first wash, it means that you need a second wash with shampoo (that's how professional shampoos work, many of them at least) and you need a minimal amount to clean them compared to a supermarket shampoo because the concentration of the ingredients it has is much higher than that of common shampoos that are diluted. Also, make sure to clean your brush or comb frequently because the reason why the shampoo may not work is that (I experienced it myself).

*DISCLAIMER:
I am not a hairdresser, I just like to learn tips and various things from hairdressers on the internet. What I say does not necessarily mean that it is correct or that it will work for you. Each person has different hair, so not all products are for everyone!️ I hope I helped