I have just received one which my son had bought for me on a recent visit to Boston but I could not get it to work. Luckily I re-read the reviews and came across the one refering about having to press so hard on the button. Having used both hands I got it to work! If I was being kind I could say perhaps it is a safeguard to prevent turning on accidently :-)

So far very pleased!
Tested at 6000' and room temperature - OK!
Tested at 9000' and 0 degrees F - OK! *
Yes the switch is a bit stiff but I expected that from reading the reviews and don't mind that at all.
* 0 F was device/air temp; the water was about 35 F. The instructions say the device should be above 32 F but apparently this thing has gone up Everest, so I don't really know what is okay - probably best to contact tech support to get the skinny on really cold weather ops if that is your thing.
I also read the previous reviews and my take is the previous issues with the water sensor and water resistance have been taked care of with the Adventurer Opti. Also, it's good to read the instructions - one reviewer said they had problems with the water not entering 'the chamber' - makes me suspect they tried to use it with the lamp cover on (!)

carried this all around the world inside a 1 liter Camelback bottle, using it to purify tapwater, rather than buying bottled water.