Julie Penner wrote an answer about MSR SweetWater Purifier System on July 27, 2011
On Denali, we of course had no water to filter, but we melted water and added drops. No problems- except the person who didn't use drops got sick. I agree with others, if I think there are other people around my water source, I use the drops too. If it's pretty pristine, say back in the wilderness, I sometimes just filter. It's a judgement call, but other people are the most likely contaminate. If they are around, so are their germs.