I believe the Seapack and Hydropack kits, the ones that use a syrup to promote osmosis, don't yield pure drinking water, but rather a sweetened drink, similar to sports drink. I recall reading something that sweetened drinks, including sports drink, aren't ideal for rehydration. Then again, gotta be better than nothing.

Maybe once things settle down in Japan there will be some new studies on the effectiveness of various filtering methods and devices to deal with these types of situations. Hopefully Consumer Reports or a similar objective organization will perform a scientific rather than empirical study.