Rick,
I have one of these and have used it once when camping It operates in two modes purifying either up to 16 oz of water (48 second treatment) or between 16 or 32 oz (90 second treatment) of clear (filter out heavy sediment first) water at a time.

Manual states it will destroy:
Anthrax, Cholera, Cryptospordium, Dysentery, E. coli, Enteric Fever, Giardia, Hepatitis, Legionnaire's Disease, TB, Poliovirus, Salmonella, Staph Infection, Strep, Tetanus, Typhoid Fever.
It implies there there are others it will destroy that aren't listed.

This product is generaly well reviewed. However the manual states the pen should not be operated if the device itself is below 32 deg. F. (of course the water could be frozen too at that point). Though it does say you could warm the pen (perhaps keep it close to the body to wam it via body heat - my comment, not the manual's).

It also says that when batteries are low, the UV light will turn off and a red LED will flash for 8 seconds. This indicates last treatment was incomplete, helping avoid a low dose, situation (if one is paying attention).

Uses 4 AA Battteries with Approximate # of treatments per set of:

Lithium - 130 16oz treatments
NiMH - 100 16 oz treatments
Alkaline - 20 - 30 16 oz treatments

Also, cold weather may affect diminished battery life/capacity.