I love Olaplex, it saved my hair. It's cruelty free, no parabens, no phosphates, no sulphates, no silicone.
I have naturally curly hair. It's low porosity, so I always had a hard time getting good results from a lot of different shampoos and conditioners. It was always clean, but tangled very easily, or stayed frizzy.
With Olaplex I now (starting using about a year ago) have super manageable hair, soft, bouncy, and no frizz. I only wash my hair once a week.
I use No. 3 as a pre-poo (pre shampoo) to prepare my hair for the rigorous shampoo. It makes my hair very soft & silky. Then I use Nos. 4 & 5, the shampoo & conditioner, which have much less water, unlike most shampoos & conditioners which are mostly water. So you use very little at a time; a little goes a very long way. Then No. 6 for my leave-in conditioner to define my curls and eliminate frizz.
If I want a straight style instead of my curls, Instead of No. 6, I use No. 9, a serum that's a great protector before blow drying my hair.
These all take very little product to work on my hair, so I'm not buying a closet full every 3 months.
The only Olaplex product I found that I don't like for my hair at all is their No. 0. It's a bond building treatment to use before No. 3. I hated this product. It has the consistency of water, so it was very hard to apply to stay on my hair. It drips all over the place. But worse, it made my hair feel like straw!! Very brittle and hard to manage. After searching online reviews, I saw quite a few that had the exact same experience with No. 0.
Sorry this was so long. But yeah, I really love Olaplex, the ones that work great for my hair.