The First Need also uses an iodine-impregnated filter to remove viruses which, for those with thyroid issues, pregnant women and those with an iodine allergy (like me) is a no-no. Unfortunately, you may not find out about these problems (except pregnancy) until you've used the iodine and gotten horrible symptoms. Been there, done that!

If you want to get rid of viruses, add chlorine dioxide before or after filtering. It zaps viruses and bacteria in a few minutes, but may take several hours to kill protozoa. The filter gets the protozoa and bacteria. The two together take care of everything except chemicals in the water, a real issue where you're downstream of mine tailings or an agricultural or manufacturing area.

I've backpacked Washington's Olympic NP coast (whose water sources all contain tannin) several times and had no problem with the water, even though it retains a yellowish cast after filtering (close your eyes before drinking). I usually use a powdered drink mix (the kind with extra electrolytes) in it, which helps with the flavor.