1:12 p.m. on June 30, 2009 (EDT)
Kmarr,
If you haven't already, read my series of 4 articles on backcountry water in the Articles section of Trailspace, the first of which is here.
What it comes down to is this (despite the "sound bites" you got above, which are a mixed bag):
1. There are several stages of "purification", none of which is perfect. For the most part, you only need to be at the "potable" water stage. This means "safe to drink" for the vast majority of people, with the probability of serious disease very low (not zero, but low). You do not need to have "purified" water - technically, this means sterile enough for surgical purposes. Unfortunately, in a lot of places (especially posts on the web), the various terms relating to level of purity and sterility are used very loosely.
2. Boiling is the most effective method of producing potable and pure water from backcountry sources. Note that this is "most effective", not perfect. Actually, you do not need to go to a full rolling boil. With the exception of a few, very rare biota, you only need to heat to "pasteurization temperature", which is 155F. You can get this at any altitude you can hike or climb to on Earth, including the summit of Everest (boiling temperature drops with altitude). Boiling does use fuel (hence extra weight), but you will boil your water for your meals anyway. It is inconvenient to boil your drinking water in addition to your meal preparation water, and it is very inconvenient to do it on a long day's hike. In 3rd World countries, there are biota (certain bacteria, for example) that survive boiling even for extended time, but these are mostly just "inconvenient" for humans. Boiling does not remove chemical contamination, so select your source carefully in areas where there has historically been a lot of mining (here in northern California, many water sources are contaminated by abandoned mercury mines, for example).
3. Filters depend on pore size for effectiveness. Most backpacking filters are effective at removing protozoa (giardia, crypto, others) and bacteria, along with most of the sand and silt (which will clog the filter quickly). Some filters (pore size 1 micron or smaller) will remove most viruses. Filters are ineffective at removing chemical contamination for the most part, though some using activated charcoal will remove some. There are no standards for removing chemical contamination via filters. Filters are heavy and require maintenance.
4. Chemical treatments (primarily chlorine and iodine compounds). These will inactivate or kill protozoa, bacteria, and viruses, though the concentration required and time for full action depends heavily on the temperature, pH (acid/alkali), and turbidity (silt and other junk). For most mountain water in the US, iodine is sufficiently effective within 15-20 minutes, chlorine in the form of bleach in 2-4 hours, and chlorine in the form of sodium hypochlorite in 30 min to 4+ hours. The big problem is that it is difficult at best to get water out of a stream, spring, or lake that is in the ideal range of temperature and pH, along with not knowing what exactly is in the water. Some sources have very little or even no protozoa in cyst form, so chemical treatment is very effective. Other sources, far into the wilderness, is heavily contaminated (giardiasis is also called "beaver fever"). Crypto cysts can be extremely resistant to all the chemical treatments with cold water and pH more than 2 or 3 points from neutrality. The chemical treatments are also ineffective against chemical contamination, including "red tide" algae that is found in some wilderness sources (usually you think of it in connection with the ocean).
5. There is another alternative that is fairly hard to find, but worth it if you are going to an area with fairly contaminated water, made by Pur, a Swiss subsidiary of Proctor and Gamble. This is in some sense a chemical treatment. It uses a flocculus to trap the protozoa and most bacteria, as well as many of the chemical contaminants. After the flocculus settles to the bottom (about 10 minutes), you decant the clear water and let it sit for another 10 minutes, during which the purification chemicals destroy the remaining bacteria and viruses. A big disadvantage is that it is packaged to treat 10 liters at a time - ok if you have a group or are in a fixed camp, but impractical for a group of 2 or 3 on an extended hike.
6.(forgot to add this, so did an edit) - I left out the use of ultraviolet light. One version of this is good for emergencies - just put the water in a transparent container and set it in the bright sun. How effective this is depends on the transmissivity of the container. Unfortunately, most glass and transparent plastic cuts down on the UV transmitted, but it does help in an emergency situation where you have nothing else. Paul Auerbach's 12 pound Wilderness Medicine book discusses this and gives references to studies that show that it is better than nothing. More effective is to use a SteriPen. There have been reports of the SteriPen failing or being ineffective. Where enough information was available and the people who had the problem could be interviewed, it has always turned out that they were not following directions, running out of battery, or otherwise misusing the gadget (most of the reports of failure are of the sort that "something didn't work right" with no details on what happened and what the user actually did in the use of the SteriPen). When I went up Kilimanjaro, it turned out that the guides and porters were using the SteriPen when refilling at streams on the way (they boiled the water at the fixed camps). I had a SteriPen myself for personal use (with the solar charger box), and I do use it a lot. The disadvantages are that it uses batteries (which can run out), and the water needs to be fairly clear (their filter does a reasonably adequate job, or a coffee filter will also work). It does inactivate protozoa (it will kill protozoa even in cyst form with enough repeated treatments, but inactivation is supposed to be adequate to prevent giardiasis and crypto problems), and kill bacteria and viruses.
7. The other methods are more for large operations or when you can transport the heavy equipment needed - distillation and reverse osmosis filters. Distillation involves evaporating the water and recondensing the vapor in a separate container. Reverse osmosis uses the application of pressure to force the water through what is essentially an ultra-fine pore filter. These are ok for use on your sailboat as you go around the world, but not backpacking. Yes, there are solar stills, but these are very slow.
Bottom line is that all the methods are somewhat effective in particular circumstances, and all have their disadvantages. I personally use filters much of the time, chemicals some of the time, boiling on short trips and in winter when I have to melt snow for water anyway (and the extra fuel to get to boiling is negligible by comparison). I have never gotten sick from the water while camping, though it is said in the wilderness medicine literature that some of us who spend lots of time in the woods and hills are carriers of giardia. Related to that is that I eat the local food when I go to 3rd World countries and haven't gotten sick from that (although I got very sick from the shrimp cocktail one time when I got upgraded to first class on one of the major airlines, as did my 4 companions on that flight).