Out in the backcountry, it is essential that you purify your water. Some people play the giardia roulette game, ie. hope for the best with unpurified water. However, when you are at a low altitude, you have no idea what type of rotting carcase or pooping moose is just up stream from where you are. I never drank water straight unless the stream was small enough to not contain any real fish, and big enough that I could dip my bottle in without drawing in any sediment, and only if I was close to the summit of whatever mountain I was climbing. However, you didn't hear that from me! Generally speaking, it's the best idea to purify all water. I was willing to take the risk for the sweet, fresh water that I was able to draw out.
When I started the trail, I had a gravity filter (you collect water in a vessel, hang it up, and allow it to drip through a purifier into another vessel). I thought that would be nice because I would get into camp, hang it up, set up camp and by the time I was done I would be able to have clean, chemical free water. However, I soon realized this wasn't the case. They are a lot to deal with, you need to have the perfect height to hang the water so the vessel collecting the clean water is on the ground. Not only that, but the filter can freeze.
Option B. Pump purification. Probably the most common of the options, there are tons of pumps, sizes, brands etc. and that is what Andy brought on the trail. It was nice to not wait for drinkable water, however, it was quite the work out to pump 3 liters just for myself. I think one time I counted how many pumps I needed, and it was about 120 -- that's a lot, especially mid hike. Additional problems include, carrying a heavy device, having to pump until your arm falls off to have enough water for two in camp, breaking components, and clogged filters. When you use it every day for 6+ liters, the filter gets used up quick. When this happens, it makes it incredibly difficult to pump, and becomes dead weight in the pack.
Option C! A relitevly new method is the SteriPen. Good for smaller hikes where you only need to purify a nalgene, the SteriPen lets you purify water without chemicals, fairly quickly and very easily. However it's not great for large bladders, and you have to deal with charging/batteries etc.
While all these options have their ups and downs, by far the best -- for us -- was chemical drops, in particular Aquamira. Most people shun drops because typically they use iodine tablets. This makes the water a brownish color and tastes pretty nasty. However, Aquamira doesn't do that. If anything, some feel there is a slight lemonly flavor. They are super easy to deal with, and you can adjust how much you use for how much water you need to purify, thus, it is group friendly. No pumping needed, and not batteries to deal with; it is also the lightest weight option out there. The only downfall to these drops are if you have a small water source -- you can accidently grab some sediment. For that, I suggest bringing a tiny sive to pour the water though. Other than that, Aquamira is by far my favorite method for purifying water: simple, fast, lightweight, and mostly taste free!-Ian