Hi 2ski2moro,
Speaking of someone who has a microbiology degree (from a long time ago) and who once worked in a lab that studied Giardia lamblia and Cryptosporidium parvum (both protozoans that cause diarrhea), I am very hesitant to use UV-pens as my only water purification in the backcountry. Two reasons: 1) they require very very clear water, and cannot permeate into particulate matter, and 2) they do not kill microbes, they only prevent them from reproducing, and even that is reversible if the water is exposed to visible light for a period of time afterwards.
Personally, I think that the UV-pens are great for travel, if you're going to be drinking/washing with tap water that you don't quite trust, but if I was in the backcountry, I would want to pre-filter my water beforehand, to remove any particulate matter. The commercial water filters are great, but can be bulky, and you have to keep in mind that they do not filter out viruses (here's where the combination of the two would be great). I'm not sure that a bandana would be good enough as a pre-filter (sorry Pinto), but it or a coffee filter might help in a pinch. Some of the UV-pen devices are sold with a pre-filter too. Also possible are the chlorine drops or iodine tablets that you can throw in your backpack or first aid kit. While I haven't really had an opportunity to go backpacking in a while (DH's not fond of the idea), I too have unpleasant memories of running out of water in the woods and always carry the tabs in my backpack if I'm going out hiking.
A couple of things for you to consider when making your risk assessment:
1) How far does the water have to travel from its source to get to where you are? The longer the distance, the greater the risk of water contamination.
2) Are there cities/farms/wildlife upstream of where you will be drinking? In the US, the water tends to be cleaner downstream of a city or town's wastewater treatment plant, but much much dirtier downstream of farms, feedlots, and wildlife sanctuaries. Cows, pigs, deer, beavers, etc. all tend to carry Giardia and E. coli and other nasty "bugs". Untreated human sewage near the water can expose you to bacteria and human viruses and parasites, including worms.
3) How cold and fast flowing is the water? Cold water tends to harbor the "hearty" bugs like Giardia, warmer water has a higher bioburden and more E. coli.
4) Have there been any storms recently? Is it springtime? Both things tend to wash bacteria and parasites into the water system. (DH got Giardia after swimming in a lake several weeks after a tornado came by, washing dirt from cattle farms into the lake).
5) How thirsty are you and how far away from clean water are you? While a water-borne illness isn't pleasant, IMHO it's worth the risk of getting a GI-bug if the alternative is severe dehydration or worse. They can always treat you later if need be!
Good luck in choosing something that will work for you.
