In Poland my mum and me went to a drug store and tried to chose one that looks promising. I've heard many good things about the Bielenda 8 in 1 nail hardener, but also that it contains formaldehyde and decided not to take it. Instead we found this 10 in 1 hardener, with Diamond powder and without formaldehyde. It sounded good and wasn't expensive either, so we bought it. I started using it straight away. so from the last week of December, applying two layers of it as often as possible.
The 10 effects listed on the pack are:
-hardens the nail
-improves the nail's resistance
-improves the thickness of the nail
-makes nail polish stay on for longer
-gives the nail a shiny look
-the nail is more even
-makes the nail look prettier
-is easy to apply
Now that I've used it for some time, I can really tell that it already helped my nails get sturdier. They got harder and are not so easy to bend any more. They do still break sometimes, but definitely not as often as they used to.
A very good point is that this hardener really dries in seconds, so I literally apply it sometimes just before I have to leave the house.
I will definitely keep using it for as long as possible, as I want to grow healthy longer nails to wear nice nail polish, but I can seriously say that I can already tell a difference, which satisfies me.