I think the Sawyers filters have revolutionized the purification process. After a lot of research, my daughter in Brazil is using one as her primary water source. They make two versions. One that does not filter virus as that is not much of a problem in most countries, and one that does filter virus.
I don't have the information in front of me so for details check out
http://www.sawyer.com/tech-water.htmlI have several of each and find them very easy to clean (backflush) and both have good flow rates considering what they are doing.
The original .1 squeeze filter, which uses a plastic bag which is squeezed to force water through the filter had a bad reputation for leaks. However in my opinion people were playing gorilla with it. At about 3 oz, it makes a very good survival item.
Two versions of .1 micron are for sale at Walmart for about $30. One is a bottle, the other the squeeze.
Nomad.