If you are double-treating, the filter takes care of the protozoan cysts (giardia and crypto) which is what chlorine dioxide needs 4 hours to zap. You therefore can cut your purification time to half an hour or less if you double-treat. However, the only reason for double-treating would be if viruses may be present, since the filter takes care of everything else.

I've found that my homemade gravity filter (described in the Make Your Own Gear section) is far lighter than the amount of extra water I'd have to carry for several hours waiting for the pills to work. The exception would be if I'm in an area that has sparse water sources, in which case I'd be carrying the extra water anyway. My big objection to the tablets, though, is the chlorine taste. I actually had to throw some water out last summer that I had treated because I couldn't stand it. BTW, I have problems with chlorinated city water, too!