This device is money well spent. I recently purchased one after reading every review (user and university) on the internet. This device lives up to the substantial hype. In response to the prior reviewer, my own solution has been to treat water in a 1L Nalgene and then pour it into my 6L dromedary for transport. During use, the water is stirred ensuring all water clinging to the inside walls of the Nalgene are treated. Only clean water is then poured into the dromedary. When finished treating, I flush the Nalgene threads with a small amount of clean water from the dromedary - this can be accomplished if done carefully in my experience even when the Nalgene is full of that last liter of clean water.
Two other tricks I've developed: First, pickup a nine dollar "joe2go?" coffee filter from the sporting goods store. It is a cup shaped metal filter with very small pores which screws into the Nalgene lid. I use it as a prefilter to remove "chunks" in water than is generally clear with the occasional pine needle or other debris. Secondly, I also have a headlamp which uses CR123 batteries (actually I use SF123 from sure-fire.com). When the batteries are low enough to reduce the output of my regulated LED headlamp to "low" I set them aside and use them in my Steripen later. Because the steripen will not "fire" if there is not enough energy to purify water properly it is safe and helps me use batteries in the most efficient manner possible.
I've used this device on several trips, purified some VERY questionable (to the degree that there was almost no question about it, I knew it was bad) water and have not suffered any ill effects. The last trip in particular I found myself out after dark and with a few friends who were using an MSR pump. Being able to relax and kick back and "stir" after that absurdly long day and still be certain my water was safe (safer even than the pumped water) was a "seal the deal moment" for me. I would give up any item in my pack before this device, except maybe the pack itself and perhaps the map. Battery performance was better than advertised with surefire batteries and if there is any doubt in your mind just read the university reviews.
This device is money.