For years I have extensively and exclusively used   
Aquatabs.These tablets are used around the world by .gov organizations, ngo's etc and have a very good track record. The tablets have a 30 minute water contact time and like any other similar products,  
the cleaner the water to start with, the better.  What I like about these tablets is that they are cheap enough ($8.75 per 50/ strip pack) and have a 5 year shelf life which means there is no reason not to carry these not only for wilderness use, but also having a partial strip pack of 10 or so in the urban PSK only makes prudent sense. 
 Aquatabs sound like a great idea. However, after a quick check, I can't seem to find anyone selling them in the US.
Does anyone know of a US source for Aquatabs?  Or, alternatively, does anyone know of a Canadian distributer who would ship to a US address for a reasonable price?
If all else fails, I get down to Seattle frequently, and while there I occaisionally go up to Victoria or Vancouver.  I suppose I can get a supply that way.