I wasn't sure what to expect so after a couple of uses I decided to measure the efficacy. After pre-cooling the bottle to the same temperature, I filled with chilled water from the fridge and then measured the water temperature at 4C. After opening, drinking and closing multiple times through a night in which the room temperature did not drop below 30C, I measured the water temperature again, 12 hours after filling. The temperature now was just under 10C. Noticeably not as cold as when filled, but still very pleasantly cool to drink. For the price I paid I am more than satisfied. I suppose my only wish would be a spout to drink from without having to unscrew the cap each time.