This has been mostly used for a trip to Nepal, it was a pleasure to use, and I imagine the main factor in keeping my completely healthy for 6 weeks.
I used it everyday for almost all of my water and I changed the batteries once. A great little thing if mineral water is hard to come by, its helped me out many a time when I have needed just a little water to brush my teeth, but cant be bothered to go out and find a shop!
Alternatives to this (things that kill viruses as well bacteria) are not terribly convincing, this is completely chemical free, and my tongue and general well being appreciate that.
I used it after filtering the water beforehand with a homemade gravity filter, as the water must be fairly clear to remain effective.
The only thing I would like to see improved is the strength of the led light which indicates the UV bulb is on; It can be a little difficult to see in direct sunlight, it is possible though, you may need to use your body to create a shadow. So not a massive issue!