Bought this shampoo (along with the Anti-Usure Conditioner and Intensive Treatments for Weak Hair) after being recommended it by a Stylist and in a very well known and posh Salon in Manchester, and, after crying at paying that much money, was surprised by how much stronger and sleeker it has made my hair. Seems much more healthy and even lasts longer in between washes (stays in shape and is less greasy).
I also alternate these products with the Keratase Nutrative deep conditoning treatment (again expensive but very good)once or twice a week and seems to have made a big difference.
My opinion of the range is that, yes, it is eye wateringly expensive (to us non-rich / non-celebs!) but it is probably one of the best professional products on the market - certainly the best that I have tried, which includes Redkin Products (a bit cheaper, good, but not as good) and Tigi (nice for normal hair but not good enough for my damaged locks).
If you use it sparingly, think it will last about 6-8 weeks at least.