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.