zerapio wrote a review of Platypus Insulator Hydration System on November 25, 2011
The classic problem, you bring water to your trip and then you take a sip and the water's frozen. I've used this product for more than a year and I can't remember having the hose frozen shut. The cap helps provide insulation to the mouthpiece and also cover it to prevent contamination (hanging hose anyone?).
Aside from the great insulation I also like the antimicrobial properties of the plastic. After my last trip of the season I forgot to take out the water (I know). A couple of months later when I noticed I just cleaned the reservoir and hose and the water tasted great. I liked the product so much I ended buying one for a friend after seeing him trying to suck water through a frozen hose.