Great... if full of faith!
If I recall, Steripen's UV light sort of "castrates" the bacteriae (I cannot get the proper word, sorry), so that you drink the giardia, but it won't reproduce inside you. If you plan to use it, it's best to have their 1 liter transparent bottle to adapt a filtered cap (for dirtier water), where the rubber right over the bulb fits, so it is easier to make the water turn around. Transparent is good to see that the light is actually working.
The device may be great... but I still kind of distrust it, just something in the back of my mind that I never felt with pills or ceramic katadyn (which is great, but so slow with small ones like mine!). It's related to a time when tap water tasted like taken directly from the toilet and I got "turista", maybe the bunch just did not have need to reproduce to make the effect! 90 seconds is unbeatable compared to a 1h pill or a 100 boring pumps, but I always wonder, is this light really killing or just lighting?