The Berkley and other gravity candle filters are OK if your water doesn't a have a lot of crude in it. If so, they will clog.
Somebody suggested using a 5G bucket and the candles. While cheaper, it has the same disadvantage. A better and MUCH cheaper solution for having safe emergency drinking water is to use the 5G bucket and some in-line cartridge filter(s). You can elevate the bucket to generate much more water pressure which then pushes the dirty water through the cartridge filters.
Depending on your selection of cartridges, you can filter out about anything.
Best place to get the parts and good prices for most filter types:
http://www.premium-water-filters.com/If Mark is still there ask him about an emergency SEGMENTED LOW PRESSURE FILTER. He used to have a kit, including the bucket with a pre-drilled hole in the bottom. I have one stashed away in my garage for "just in case."
You'll also find a lot of great information and various water related links on the Premium Water Filters site.
BTW, most filters, including all candle filters, won't remove a virus - like pandemic bird flu. Reverse osmosis ones will but they require energy and waste too much water. And they are expensive. However, you can simply add a little chlorine (like PURE unscented Clorox) to kill all virus. A solid carbon block filter will then filter out the chlorine. Premium Water Filters also carries a "nano-ceramic" filter that is supposed to filter out a virus. You can put that one on your regular house water to prevent an "oops" contamination but if H5N1 infected birds are swimming in your emergency water source add chlorine on the "better safe than sorry" principle.