We use the steri pen. For many years, I used iodine, or occasionally bought water. In comparison, the steri-pen has been a miracle worker.
It was the most help to us in Nepal, where all the water had to be treated, but was quite clear to start with. We did a 2 week trek to Everest Base Camp using the steri-pen to treat every drop of the water we drank, and never got sick. The pen is small and really lightweight (2 oz), so we didn't mind carrying it. It sterilizes 1/2 liter in 45 seconds or 1 liter in 1 1/2 minutes, so it was fast. And the only thing required to keep it going is batteries - no filters to change. We were able to do all our own water as well as water for other travelers.
When we got home, we used it in the Colorado Rockies. Getting to drink cold mountain water minutes after we collected it was amazing.
Just be sure you'll be able to get relatively clear water. If the water is too muddy, the light won't be effective.
I spent a while in the countryside in ivory coast, and at first, I only drank bottled water.
But really there are plenty of ways to make sure you don't get sick.
First: assess the risk. What water are the locals drinking? Is it from a well? Is there a pattern of people getting sick? What do they think? I've been in communities where I just ended up drinking the well water and never had a problem, because I became pretty sure that was the safest water around. Turns out the locals bottling the water didn't do so in the most sanitary of conditions. I only learnt that from locals.
Second: Iodine tablets. If you need to have water, yet you don't know of any reputable source, iodine tablets can work well. Some filters are pretty well rated too.
Third: Consider water that has been boiled (like tea, not caffeinated - caffeine tends to dehydrate you), or sodas. In Ivory coast I could always buy a coke, even in the remotest of places. Super unhealthy in the long term, but it won't make you sick right then and there.
If you're planning a trip, don't worry about it too much. Maybe bring a filter and iodine tablets, then use your best judgement while there. The more experience you get the less you'll even think about it.
I've become a recent convert to Aquamira drops. These are just two small doppers, which when combined create chlorine dioxide and kill all bacteria in your nalgene. It realy is SO easy to use and as portable as it gets. The taste is good too.
I agree with previous posts on iodine tablets & filters. Additionally, when I was in Madagascar with the Peace Corps, every little store or stand in almost every town/village sold a product called Sur'Eau, which was a sodium hypochlorite solution (diluted bleach) that disinfects water making it safe for human consumption. Peace Corps supplied us with the product and our Health Volunteers helped promote the use of this product throughout the country. It was still recommended to filter the water first, but on a number of occassions, I simply used a drop or two of this product in my 1.5 liter bottle!
I had this water bottle that had a filter that you sucked the water out through a straw, It was great for travelling around the world and reducing the amount of plastic water bottles I bought and wasted.