Katadyne Pocket is the way to go if you want a filter that won't fail on you.

It's used by Doctors Without Borders, The Red Cross and numerous military groups. It DOES not filter out chemical nasties but then again neither do most filters. The charcoal they contain filters out some impurities and taste but will not filter out nitrates, fluoride or most industrial contaminants. Even if you could find one that does (like the British Berkey) it isn't backpackable and the elements run out in 6 months or so (sooner if the water is highly contaminated).

Note: If you purchase the Katadyne pocket throw out the abrasive pad that comes with it and use something FAR less abrasive for longer filter life. You can even use a piece of cloth if the filter is still wet.

I got stuck for 2 months in the wilderness and it functioned perfectly for the duration. I'd bet my life on it again.

-Nim