Use this filter mostly in the eastern seirras the water up there tastes great out of any stream or lake if used in "dirty" water i pre filter to get all the chunks ad stuff out. Other than that overall a great system never had any problems unless you count the time i forgot the batteries!

I bought the SteriPEN Adventurer Opti Water Purifier by Hydro-Photon, Inc. in early 2011 for backpacking the AT. I also bought the SteriPEN FitsAll Filter. Do yourself a favor; if you get the SteriPEN Opti (which you should), get the SteriPEN FitsAll Filter too. You can thank me later.
For the first four to five weeks, I used the SteriPEN every day--at least once and sometimes two or more times each day--without any issues, to sterilize my drinking water. Then, one day, rather than the blinking green light--which lets you know that the water has been sterilized--I instead saw a blinking red light. Uh oh, I thought. It turned out that it just needed a replacement battery, which, naturally, I was carrying with me.
However, about ten days later, the same thing happened--flashing red light--and no matter how many new, fresh batteries I put in (which, by the way, are rather expensive), the SteriPEN kept flashing red.
Luckily for me, I had a cell phone and a signal. I was able to call my daughter, who texted me back with SteriPEN's phone number. After a very, very long and incredibly frustrating conversation with a tech at SteriPEN, they agreed to exchange my non-functioning unit for a new one, which they sent up the trail to my next maildrop.
I was fortunate enough to be hiking with someone who was carrying a famous brand-name pump-style water filter. With trepidation, I used only this water filter until reaching my next mail drop. I say "with trepidation" because this particular filter physically removes only particles, protozoa and bacteria down to 0.3 microns in size. Unless you boil or chemically treat water from questionable sources, an "absolute 1 micron" filter is required to ensure that the water is safe to drink. Though I never became ill from drinking the filtered-only water, I would not have wanted to rely on that method for the duration of my hike. I consider those who do to be taking risks that I simply prefer not to take.
I also had chlorine dioxide tablets with me as a back up, but I prefer not to introduce chemicals into my body unless absolutely necessary. I just don't want you to think that I was otherwise unprepared. Had I been alone, I would have chemically treated my water until I could resolve the problem with my unit.
The SteriPEN Opti takes slightly longer to work than a pump-style filration system, but you can have 100% confidence in the water quality. I don't know about you, but when I'm in the backcountry, that makes all the difference. There is really nothing that is as important as safe drinking water and that is what the SteriPEN Opti provides. Factoid: sterilization via ultraviolet light is how most municipal water supply facilities in the US treat water to make it safe for drinking. You should know, however, that water purified using ultraviolet light cannot be assured to remain safe indefinitely. Do some research regarding this method of water treatment to determine whether or not you may want to take additional precautions in terms of residual (chemical) treatment. The only other method of water purification that is 100% is boiling your water. On the trail, that translates to much more fuel consumption, not to mention the precious daylight that you would also be burning up in an effort to treat your water.
The SteriPEN folks did exactly what they said they would do: they sent me a new SteriPEN Opti; they sent me a return shipper, with which to return the non-functioning unit; and they pre-paid the return shipper, because to charge me for shipping would not have been fair. That said, I still cannot tell you how frustrating the telephone call was with their tech person (though I'm sure he had the best intentions). If my unit had been functioning--as it had been for over one month--there is no way that any reasonable person would go to all the trouble to find their number, call them from the AT and spend about 25 minutes ON A CELL PHONE WHICH YOU HAVE NO WAY OF RECHARGING ON THE TRAIL, to make a frivolous or unnecessary call. They should have taken two minutes to be sure I had followed all proper use instructions and then just done what they finally did anyway. In their defense, they may simply have wanted to resolve the issue for me right then and there, if possible, so that I wouldn't have to spend even one more day without a working unit.
When all is said and done, I am still happy that I bought the SteriPEN Opti. If I had it to do all over again, I wouldn't change a thing. I would still buy the SteriPEN Opti. I would, however, be much more diligent about keeping my SteriPEN dry. I cannot be certain, but I think that water may have gotten inside the unit somehow, which may have been what caused it to malfunction. I believe that the Hydro-Photon people are first rate, if a bit geeky. Then again, you want geeks designing your water purification gadgets.
If you are looking for a false sense of security and speed, go with a pump-style filter. If you are looking for safe drinking water, lightweight, good customer service and peace of mind, get the SteriPEN Opti. One final adviso: do not purchase the solar recharger for backpacking. The solar unit is about as heavy as a brick, and, if memory serves, about the same size. I sent mine back and I have never been sorry. Buy and carry a spare battery; it should be all you need.
If the Hydro-Photon people fix whatever the issue was that made my unit malfunction, I could give this unit five stars. Heck, I could give it ten stars!

JUNK, JUNK, junk,
If you want to waste your time this is the item for you. Shoddily made imported garbage should NOT make it onto R.E.I.'s shelves. But IT DOES. This is so poorly made it didn't even survive opening the battery compartment. I wouldn't give this to an enemy much less a friend. When I tried to register the item online the prompt required me to open the battery compartment to check the serial number, which wasn't there. Upon reassembly I attempted to replace the screw top cap. The cap wouldn't close or allow the battery compartment to be opened at all. Total shoddy junk. It seemed like the assembly from a 2$ l.e.d. flashlight I own, NOT a near 100$ water purifier. If it cannot survive the battery compartment being opened ,what makes anyone think this toy will save you from impure water. Items that are lifesaving tools should NOT be made poorly. Great IDEA. EXTREMELY poor design. If I could have given this item 0 stars I would have. Don't waste your time. Get a "real" water "filter". Not a shoddy toy.