I have just returned from a trekking holiday in Greenland and found these tablets to be invaluable.
I know that some people say that the water in the streams and lakes is completedly safe to drink, however the sheep dropping and green slime in some places made me think otherwise...
The important thing is that, when doing strenous walking, to drink enough. For a 10 hour day, carrying enough water isn't really an option, so it is necessary to pick up water en route. These solved that problem.
I do agree that there is taste of "swimming pool", however this is the lesser of two evils, the other being the effects of dodgy water (I'll leave that to your imagination).
In my opinion, essentail for being out where the quality of water cannot be guaranteed.
One point of note, and I'm not sure if this is true of not. I have been told that UV light, i.e. sunlight, can stop these working. I was taught that, if you use these, you must use either an opaque bottle or put your water bottle inside your rucksack; a transparent bottle in a net pocket is a no-no. As I say, I have been taught this, but I have no way of knowing if it is correct, but just to be on the safe side, I keep my bottle inside my pack until the waiting time is over.