I recently came back from a seven-month RTW trip. I brought along a Steripen Adventurer and a Nalgene hard plastic bottle because I didn't want to buy bottled water at local stores every single day. The countries I visited were Portugal, Poland, Ukraine, Belarus, Russia, Mongolia and China. Municipal water in these countries ranged from fairly clear to crystal clear -- but not potable.
I filled my water bottle with tap water taken from hotels, hostels... and sometimes disgusting bus station bathrooms. Not once did I get sick the entire trip!
The Steripen is effective, easy to use, energy efficient, and dependable. Stirring the water for 45 or so seconds (20oz bottle) was pretty painless -- and I went through just two sets of batteries.
For purifying water that is fairly clear and OK tasting -- I highly recommend the Steripen. Truly a great device for traveling.