I recently bought one called 'nail Life nail revitaliser'...it is only available in SALLY Beauty stores - and maybe online. I read some revues for it on CIAO...and they were quite favourable. It cost £6+ for a LARGE bottle...aprox 30ml.So far my nails are in somewhat better condition than they have been in a while.
I have also read that it's a good idea to switch products, as your nails will get used to something after a period of time.
OPI Nail Envy is also good....but very pricey.
I use OPI Nail Envy, you can buy it from QVC which is much cheaper in the shops. They also do duo bottles, so you can get 2 bottles for about £16, whereas in Boots it costs more than that for 1 bottle! I use it all the time and wouldn't be without it! It is worth the money if you keep applying it every other day, you will see the results within a week or so.
I don't use nial strengtheners but my mum bought me a Nevo dead sea spa manicure set at Xmas. My nails had been really brittle and discoloured and in a hell of a state really, but after using this just once I saw a huge difference. I've now been doing my nails once a week and they are so strong and healthy and shiny now. I think it was quite dear, but when I run out I'll def. be buying more.