My favorite size bottle. Enough water for a hot yoga class and fits in my mountain bike cage too. Not too heavy to take outside so I can have an ice cold drink on the trail or hot tea on a winter climb.
I'm not a gadget guy but this is a truly practical travel accessory. It compresses to a little 3 oz ball, so you can toss it in your luggage just in case. Great for unplanned day hikes, fishing trips, or a walk around town. Don't be that guy with a bunch shopping bags with stuff falling out as he tries to cram them into the overhead.