I bought one for myself at first. It's cheap and good. When I tried it, it had more cooling power than I expected, and it stayed cold for a long time. My husband was using a water bottle with weak cooling power, so I gave him one. My husband was delighted with it and uses it all the time... I, on the other hand, feel a sense of loss. I resisted for a couple of days, but I couldn't give up and went to the store to buy two of each size (530ml and 700ml). It looks like it will be useful when I go on long trips. I used to carry a small cooler box around with me, but it was heavy and tiring because I had to use a lot of cooling agents, so from now on it will be a piece of cake. The simple design makes it easy to wash. The outside is non-slip and easy to hold. The color range is limited to white and black, but you can use a cover for PET bottles, so you can easily identify your own bottle.