I have the Steripen Journey, which I prefer to all the other models. It weighs in at 4.5 oz with the batteries (Buy the streamlight 12 pack at Amazon).

A few features that make it superior to other models

#1 - the ribber neck around the light fits water bottles perfectly (not nalgenes) I carry a 1 liter Aquafina bottle and a platypus collapsible. The pen fits in the mouth perfectly, just turn it upside down and slightly swirl.

#2 - the LCD display tells me how the battery is doing

#3 - ease of use, the scouts can't mess up doing the process.

You do need to "blow" on the water sensors after each use to make sure they are dry or you might get the frowny face on the screen.

I too evaluated a pilot Opti unit for Steripen. It was used for 23 days on the JMT last July/August. It worked just fine. It was less temperamental, by far, than the traditional Adventurer that I own. I did use the flashlight features a couple of times, but principally it was used for sterilizing water. It is also an improvement in visibility in the bright sun but still not as visible as I would like it, one has to create a shadow/shade setup to clearly see that the unit is operating while stirring if done in bright light.

We used for our sterilizing container the 1.9 oz ZipLock Quart Container I have posted about elsewhere on BPL forums. Steripen's support said such a container would work (they did express a caveat about durability in sunlight over time, as I posted their response elsewhere).

I wondered if the glass chamber bulb was orange instead of transparent if that would not be better?

One of the other hikers had the Journey Steripen and we were somewhat jealous of the more affirming visual feedback of success, the one downside of the adventurer, is that if you don't keep your eye on the unit and have artificial shade setup for watching it, you won't realize if it operated successfully - but with the more clearly visible smiley face symbol on the Journey, it's not so much a problem with it. It's too bad there can't be a separate LED light for "successful" and one for "failure" that stays lit for 10-20 seconds and doesn't require shade to see it. I like the fact the Adventurer is slimmer and weighs 1 oz less than the Journey.

It definitely is an improvement over the older Adventurer. The flashlight feature is nice in a pinch too.

Used an Adventurer Opti in August 2011 for about 20 quarts, then stored unit with batteries out of unit in separate ziploc until Sept. 2012. Re-inserted batteries and attempted to use unit--unfortunately, not before hitting the trail. Unit would seem to turn on with one press, small sensor light and green indicator LED would begin flashing, but as soon as I put the unit in water, the indicator LED would turn solid red and the sensor light would shut off, with the UV light never having come on.

Back home, measured battery voltage, and both CR123's at about 2.89V.

How low can battery voltage go, and unit still work?

Shouldn't the batteries have maintained their voltage longer? (Steripen claims 50 liters). Tried cleaning contacts (which looked good), tried warming batteries (ambient temps 50 F+, not cold).

“Back home, measured battery voltage, and both CR123's at about 2.89V. How low can battery voltage go, and unit still work?”

Just some thoughts:
1. What was the brand?
2. How were the batteries stored? Hopefully at room temperature and moderate humidity. They don’t do well when stored cold or hot.
3. 2.89V is a good sign your batteries are shot. It’s not low enough to turn the LED initially red. But when you stick it in the water, all of a sudden a near Amp is being sucked from the battery and the voltage will drop big time and thus the UV tube cannot turn on.

-Barry

If you use Steripen, make sure and bring spare, unused batteries. Use brand name, non-rechargeable batteries.

I've had same experience with less use and less time between uses

Actually, I just returned mine for the exact same symtoms. Rich, the service person, said it sounded like a bad bulb. The tech sent back a scribbled note stating "circuit problem." The old batteries that I was using, did indeed work when it came back. I tested it with three sets of batteries: the old set, and, 2 new sets.
All did the same thing you describe before the repair. BTW they sent me a new one.

Thanks everyone for the very quick info. I've also emailed Steripen, and am waiting to hear from them.

The batteries were Energizer lithium CR123's stored at room temp (55-85 deg. over the course of the year, probably) in their own Ziploc. (They were the original batteries that came with the unit.)