This one? I have naturally (very) curly hair and have used this in the past. It does work, although if I wanted straight hair I still needed to blow dry and GHD it to within an inch of its life after every wash. This treatment just made it possible to do this so it would actually go properly straight and stay that way rather than go fluffy and/or instantly curl up again. If I washed it and left it to dry naturally without blow drying and GHD-ing it would be wavy rather than curly.
It's not exactly easy to do, you really need someone to do it for you. Top tip is to do it outside as the fumes when you're blow drying the treatment in are pretty bad - get an extension cord for your hairdryer! It's quite time consuming - wash hair first, blow dry, paint on treatment, wait, blow dry in (takes AGES) and then straighten tiny sections over and over again (takes AGES) to seal it in. Then no washing or getting hair wet for as long as possible, minimum 3 days but the longer the better.
I probably used it for 2 years in total, re-applying every 6 or 7 months, but stopped about 18 months ago as the constant heat from drying/GHDs was so bad for my hair. I had to cut a couple of inches off the length as the ends were totally frazzled; I've gone back to embracing my natural curl and my hair has grown again and is in much better condition. If you don't need such heavy duty straightening it does work really well though. I might use it again at some point but not straighten regularly afterwards, just use it as a relaxer type thing and have the beachy wavy look which I quite liked.