I have used a Steripen Adventurer for three years now and feel that it works as well as any of the water treatment systems available to backpackers. Of course nobody on this forum can tell you with any accuracy that it (or any other system) works: all they can say is that they didn't get sick when they were using it. That is all that I can say as well.
The Steripen has a few limitations. First, you don't want to use it with water that is turbid or otherwise cloudy; the murkiness limits the effectiveness of the UV source. Second, you will want a backup set of batteries and water treatment chemicals. Third, you want GOOD batteries in the thing. I use nothing but lithium ion cells and get good battery life and reliable low temperature operation. Rechargeable and alkaline cells are not reported to work either as well or as reliably. Fourth, you want to read the instructions thoroughly and carry them with you as well. I had a problem with mine on the JMT last summer, a problem that was quickly solved by reading the directions. Fifth, some of the Adventurers come, or came, with a stiff switch button which can be difficult for aging fingers, such as mine, to operate. I have adapted to the switch and also it has loosened up a bit over time; some folks have not been able to work it and I can understand why.
The lightest and most reliable system though is chlorine dioxide tablets such as Micropur. A 1-week supply weighs less than an ounce. The only drawback is the 4-hour kill time for Cryptosporidium if that is a potential issue. Also, there is a faint residual chlorine flavor to treated water; swimming pool flavor if you will. If you like the taste of cold mountain stream water then the ClO2 tablets ruin that. They are convenient though!