I've met a lot of people on the trail who swear by the Steripen. I must say, it is a great concept, however, I've seen a significant number of trail journal entries speaking to unreliability. Before everyone who's never had a problem chimes in, that's not the definition of reliability. The ratio of those who've had a problem to those who haven't is a better measure, and by that scale, a Steriipen doesn't have a great track record. Batteries are another consideration.
I currentl chemically treat my water for viruses and filter for protozoans (and to remove the virus killing chemicals after they've done their job. I'm seriously considering the Sawyer viral (0.02 micron) filter. No chemicals, no batteries and the manufacturer claims a 1 million gallon life expectancy. I've been using the Sawyer 0.1 micron filter with no problems for two years. The 0.02 micron filter would let me ditch the virus chemicals and the charcoal filter that extracts them. The initial price is higher than the Steripen, but no batteries and high reliability would seem to make the Sawyer a better value.