I opted to purchase the "Opti" version of the Steripen, as I've heard it's the newest version that is less problematic. It uses an optical sensor to detect the water. The older versions (aka Steripen Classic) use a couple of metallic pins checking for electric continuity. Some of the problems arose when a water source wasn't saline enough. The pins don't get continuity, the UV bulb won't light. Some report workarounds by adding some salt/Gatorade to the mix to have it work properly. Again, it's only for certain water sources, but I didn't want any fuss.
There is 1 button on the unit. 1 press treats 1 litre, and 2 button presses treats .5 litre. I have only tested at home in a glass of tap water so far. I don't know how the optical sensor works, but when I dip it in a glass of water, the UV bulb lights up and begins the process of blasting the water. You simply stir the water lightly until the light goes off and a green LED indicates the process is complete. If for some reason the LED lights up red, the process was incomplete, and you'll need to re-treat.
Semi-turbid water can be treated, but it takes two complete cycles. You have to use your own judgement as to what is turbid or not. I also purchased the pre-filter that goes over the neck of a bottle and has a 4 micron screen to keep bigger floaties out.
Negatives are that I actually have to treat a wider-neck bottle. It will not insert into a narrow-necked 'soda' or water bottle, (the Classic model does, however). A Gatorade bottle works fine -- and even mates perectly with the pre-filter. Would also work great with Nalgene bottles. The Gatorade bottles are light and durable, so that's what I'll stick with for my use. It also cannot be used to directly treat a Camelback/Platypus bladder. Rather, you'd have to treat a bottle-ful and pour into the bladder.
It uses a pair of CR123 camera batteries. The unit came with a pair of Energizer brand cells. I've ordered a 6 pack to have on hand, and will take a spare pair with me on the trails. I'll still be keeping a backup method with me, whether that be chlorine or iodine solution. I'll update as I use it in the real world, and report any concerns or findings.