Having had giardiasis, I can tell you, it is not a mild infection in the least. I also suffered from the treatment as the antibiotic (flagyl)I was given also wiped out all the probiotics in my system. In a survival situation, giardia and crypto are a big problem.

Chlorine effectiveness diminishes over time, as the chlorine compounds break down and become inert. Also, adding a little lemon juice or vinegar makes the chlorine used more effective as lowering the pH will make many of the microbes more susceptible. Warm water will also increase the effectiveness of the chlorine application.

I prefer to boil suspect water whenever practical, use filtration in most wilderness settings when boiling is not a viable consideration, and rely on chemical treatment only when such treatment is necessarily expedient. While biologics are a problem easily dealt with using these methods, chemical contamination presents some serious challenges. I have a couple friends who've dealt with arsenic poisoning from drinking untreated water.

Distillation would be the most preferable process for cleaning up water, but it too is not entirely foolproof.