I've used this on and off for a few years and bought several bottles, applied it in different ways, etc. I think the product is great but it's good to know that it doesn't compensate for an otherwise bad routine. However, in combination with a good shampoo and conditioner, it's top notch.
Like many others, I don't love the original way of applying (shampoo, not conditioner). I felt the wow effect when I started applying conditioner after K18.
My current application method is: shampoo, rinse out, wring out as much water as possible, K18 for four minutes, then conditioner and then rinse everything out. I have such thin hair that leaving it in doesn't work well; no matter how little I take, it gets sticky. Still, I notice a big difference using it and that the result gets better over time. If I had more patience, I would also towel dry before applying, but honestly, I usually don't.
If you have hair like mine – Nordic, thin strands, length between shoulder and bra strap, etc. – my recommendation is to buy the smallest tubes to start with. That will be enough for the first treatment, and then you can decide if it's worth buying bigger ones.