This is a link to the findings of the Stampede trial last year: http://www.ascopost.com/News/27615
You will see that on average, men with metastatic disease (and on HT) who received early docetxel survived for 65 months rather than 43 months without it. That's where the "two years longer" comes from: It's two years on top of an already very limited life expectancy, which sounds like a worthwhile benefit.
But an average is only that; it's not a minimum, and it's by no means guaranteed. My husband Tony had early docetaxel last year, finishing in March 2015. When these trial results were published in June, we were delighted, and hoping for several years more of good quality life together. However, by July his PSA had started to rise again, and by September it was obvious that the HT was failing to control the cancer. Enzalutamide brought several months' respite, but now the cancer is in his bone marrow, making further treatments (including further chemo) pointless. So, 16 months after completing the docetaxel, he has only palliative care from now on, and only months to live. For what it's worth, Tony tolerated the chemo very well, with few side effects (and arguably few effects at all!)
I'm sorry if this all sounds gloomy, but look at the positive side of this lottery: for every man who gets no benefit from docetaxel, there will be someone else who gets years of extra life from it. You could be lucky, or even average, either of which makes the chemo well worthwhile.
Marje