LOVE these for hiking, biking, camping, backpacking... There is the obvious weight and space savings as you drink and it gets smaller. For a REALLY long and/or hot day, I will fill my 3 liter hydration pack, AND throw in one of these so I have 4 liters total. The only downsides that I have found so far are that I tend to lose the caps (but I'm sure that's more my fault than Platypus'), and if you are backpacking with a Steripen, it does not work with such a small opening. So you still need a wide mouth bottle if you're planning to sterilize with the steripen. One nalgene and multiple platy bottles will do the trick though.