So why are filters so popular? Lots of reasons. For one, they allow you to fill up with water while on the go, hiking or biking. No stopping, setting up a stove, and boiling a quart so you can have something to drink. For another, you boil water and you have... hot water. Great for coffee, not so great for a cool drink on a hot summer day. Boiling also means you must have a stove, and maybe you're just day hiking and don't want to hassle with that.
Meanwhile, filters also do something that boiling cannot: They remove many waterborne chemicals and even filter out "off" tastes.
So there you go. I boil water as a means to purification when the boiling has another purposemaking dinner, or brewing coffee. Otherwise I use a filter. To some degree, which filter is a matter of preference, but I've long usedand likedthe MSR MiniWorks, now in an improved design called the MiniWorks EX ($80; www.msrcorp.com). Sturdy, easy to use, effectiveat least, I haven't contracted anything after using it!
Check out Outside's 2004 Buyer's Guide for more on water filtration and purification.