I've got a steripen adventurer. When it works, and used on clear water, it can't be beat. I've never had water taste so good, and had such an easy time purifying. No wait, instant clean water.
Having said that, I'd never go out without backup (aqua mira loaded into the small droppers available on BPL). I've had the thing fail on one trip for no apparent reason. It's worked again since. I've heard from others that it is more likely to malfunction using rechargeable batteries, which is what I use.
It does need a wider mouthed bottle. There are some lighter alternatives to a regular nalgene that will work (nalgene cantene, gatorade bottle), though none as light as a platypus or plain H2O bottle like a smartwater.
If you're dealing with water that's murky or has a lot of organic matter the water has to be filtered a bit, and the water tastes pretty bad. OTOH, same is true with chemicals.
To keep the batteries from draining you need to take them out after usage. The unit continuously drains the batteries when they are inserted. They will drain completely after being in the steripen after a couple of weeks even if it is not used.
If you decide to use lithium batteries for cold weather conditions or reliability considerations, it is not at all cheap to keep picking up two pair of CR123 batteries for each week out.