Originally Posted By DTape
Chris gave a good summary. The only pro I can add to his list is speed. A uV purifier is extremely fast from start to finish. Chemicals take time (giardia and crypto cysts require time measured in hours), and filters have more setup, maintenance, fiddle time etc... The other con to uV (also for chemical) is it doesn't remove other non-microbial contaminants as some filters do.

I got a Steri-pen for Christmas and have tested it off and on all winter with my stash of Nalgenes that I keep in the truck for use at work. (unfrozen, which was easy in this record mild winter)

I'm coming to the conclusion I may need to carry both Steri-pen and filter on some trips. But most of the water I find will be clear enough for the Steri-pen.