@Philreedshikes It is not their "thinness" but the plastic, PET,  they are made of that makes disposable unsuitable for hot liquids.  PET starts to deform at around 130F.   In a survival situation you can certainly put warm water in them to help keep body temperature up.   While I have used the "hot water bottle" reason to justify carrying a Nalgene into the Sierras where it not uncommonly drops below freezing at night in the summer there is certainly no reason to carry more than one and then only if you expect temperatures to fall below freezing.  Having camped in snow and been too lazy to bother,   I no longer find that reason compelling for 3 season use.  Taking a few hand warmer sachets is to take the edge off seems a better use of the weight. 

@beckyt despite the not-so-positive reviews from others users in this discussion, I actually love the Geopress. I bought mine about a month ago and have already used it on 2 backpacking trips (a 3-day trip and a 4-day trip) in the U.S. I use the 24 oz Geopress.

Some people may say that the likelihood of finding viruses in U.S. water sources is low, they have been known to exist, and it's always better to be safe than sorry! The safest bet is to get a water filter/purifier that not only removes/kills water-born pathogens (viruses, bacteria, protozoa, etc.), but also sediments, dissolved organics, chemicals, pesticides, and metals (You never know what could be in the water you are drinking!). The Geopress does all of this. 

Moreover, I actually find the Geopress to purify water quite quickly and easily (it's not hard to compress at all). I have only used one purifier cartridge so far (although I brought a second one with me on the trip just in case, as this was my first time using the product and was traveling in an area where the water has a lot of sediment and tannins), have already used it 37 times, and find that the filter still works great.

Additionally, each purifier cartridge is rated to work for approximately 250 L (350 cycles/uses), which is a lot of water, especially if you are trying to filter for numerous people, as I was. One thing to keep in mind is that the amount of sediment/dissolved organics in your water sources will affect how many uses any single water filter/purifier will give you. This is why I recommend you always bring a back-up cartridge/filter, no matter what system you are using!

The Geopress is also cheaper than a lot of other purifiers, although it is not cheap, but it's definitely a bang for your buck! You are paying for quality. Moreover, if you are purifying water for numerous people, this is a very inexpensive method! Each cartridge costs $30, which initially seems a bit expensive, but when you calculate how much water ends up costing per gallon using the Geopress, it's actually quite cheap! $0.46 / gallon, to be exact! (One cartridge purifies 65 gallons and costs $30. By dividing 30 by 65, we get the cost of water/gallon via the cartridge, which = $0.46). This is even cheaper than buying a gallon of water from the grocery store!

While it is a bit heavy, I do not find this weight to be significant enough to worry about. Besides, it carabiners great to the outside of your pack if you are short on space! Grayl also makes an ultralight version of this purifier if you are concerned about the weight.

The design of the bottle also makes water contamination extremely unlikely. You can also store filtered water inside the bottle if you want to maximize the amount of water you are carrying!

I also love the texture of the outside of the bottle - it is so grippy that even when your hands and it are wet, you won't drop it/it doesn't slip!

I plan to use the Geopress when I travel internationally as well, but personally I find it to be a great water purifier option for backpacking in the U.S. I have used several other filters in the past, but would rank this one above all others that I have used.

Lastly, another awesome thing is that Grayl has a recycling program for the Geopress purifier cartridges, so you can reduce your carbon footprint! 

www.brynsharpphotography.com

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That's great info @bryndsharp !  Thank you.  I actually had just recently purchased the Grayl for the same reasons you mentioned, but haven't actually used it yet.  Where I live there is a lot of opportunity for waterways to become contaminated by agricultural run-off and roadways, so I wanted to have as much purification as is possible.  Like you, the weight isn't really concerning as I'm not an ultra-light backpacker, nor a thru-hiker, so for all-day and 3-4 day hiking/camping trips, the weight and size will pose no issues for me.

For anyone who hasn't seen it yet, the REI Labor Day sale has all water filters and purifiers at 25% off, so now would be the time to get whichever best suits your individual needs. 

Superusers do not speak on behalf of REI and may have received
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@bryndsharpand others,

Don't get me wrong, the Grayl Is a good water purifier! As I said, I saw it years ago and it was impressive back then, what I meant was, someone asked them what would happen if they processed a Coke. Admittedly, they said they didn't know, so they tried it. IT WORKED! The liquid that came out was perfectly clear!!

However, as I've ALSO said, there's a difference between water purifiers and water filters, such that they are two COMPLETELY different categories!!

What KIND of water purifier you need depends on what you want to do with it (yes, only purifiers remove dissolved solids). Need to purify sea water? Then you need a desalinator. Other types of dissolved solids require purifiers rated for those uses.

But again, you have LITTLE to worry about in 'first world countries like the U.S. Here, a filter is all you should use! Even then, though the likihood of giardia or crypto' infection is unlikely, whether it would be a serious problem for you depends on whether you have some preexisting condition, OR, if you can't go home for more than a about week (i.e. if you find yourself in a survival ordeal). Then, dehydration due to diarrhea is likely.

In any case, any purifier, including the Grayl, is objectively overkill. But, buy what you want. If you're NOT expecting to face virus-infected water on a hike, you'll get FAAAR more "bang for your buck" with a filter like my Sawyer Micro!!!