While my wife and I used this for 5 weeks while backpacking across the country this past summer, we found the batteries dead at least once. Thankfully we had a backup plan, but it got me thinking: why bother with this, if you're going to carry a backup plan?
Yes, the SteriPen can purify 1L in 90 seconds and we never had any problem that appeared to be related to water, but for something this fundamental reliability is key. With a chemical treatment you know how many doses you have left. With this, you have no idea if you have 100L or 1L left, and who knows, if the button gets hit in your pack or the temps kill the batteries, your new batteries may be dead tomorrow morning.
There are plenty of chemical treatment options that can purify your water very quickly. Many are as light or lighter than this and you know exactly how much you have left.
I like the steri-pen, I really do. But if I'm going to carry a chemical backup plan, why am I carrying this in the first place?