I have very weak, thin nails and this product did not work very well to strengthen them. I followed the instructions exact (apply the strengthener every 3rd day for 7 days in total, remove then repeat the process again).
I did notice on the first day (coat one) that my nails felt harder than what they did without it but by the time I added the second coat (day three) my nails didn't improve in hardness and the nail treatment started to bubble on my nails.
Using this product with the intention to solely act as a nail hardener it does not work for me but I do find it alright to use under nail varnish as a base coat.
For anyone with nails like mine (dry cuticles, weak paper think nails) I would not recommend this as a nail hardener but it is only 8.00 so it is cheap enough to have a go to see if it works for you. Worse case just use it as a base coat :)
I quite liked this, but you can't expect it to work miracles. It's good, but it only works if you consistently apply it. With that being said, it is good for an undercoat and things like that.