Product review (1 stars): How to clean and reuse them, over and over
11/20/15 UPDATE:
THIS PRODUCT HAS CHANGED and my original 5-star review is no longer relevant.  I've purchased humidifier filters from Filters-Now in the past that have been just fine, but their "aftermarket" manufacturer has changed materials; these new filters are really, truly HORRIBLE.  During this month I purchased two kinds of their filters: this one (Kaz WF1 Humidifier Filter - aftermarket) and also a Holmes HWF72/HWF75 Humidifier Filter (also aftermarket).  BOTH kinds are using some kind of new, petroleum-based plastic that smells TERRIBLE.  I'm very sensitive to TOXIC CHEMICALS and can't use any of these filters.

ORIGINAL, 5-STAR REVIEW of my 3/17/14 and previous purchases:
These filters are good but they are costly to replace, so I like to keep them as long as possible.  I've got two I've used every day for a year now, and they're still going strong!  Even though I add a demineralization cartridge to the water, I still eventually get a crusty, dark mineral buildup on the filters.  With just two filters, you can remove a dirty one, wash out your humidifier, then immediately add a clean filter.  Here is my cleaning method for the dirty one, for which you'll need: 1) wide mouthed jar as tall as the height of the filter (large peanut butter or mayonnaise jar works), 2) 1 quart of white vinegar, and 3) a plastic funnel.

I stand the wet, used filter on a dinner plate until it dries thoroughly.  Then, roll it up and insert it into the jar.  Pour vinegar into the jar until it covers the top edge of the filter inside. Let sit for 30 to 60 minutes, depending on how crusty the filter was (you'll see the minerals dissolving and foam at the top).  When the filter looks clean, use the funnel to pour the vinegar back into the quart bottle, as you'll be able to use it again many times, topping off with fresh vinegar when the level in the jar begins to drop. (Keep a 2nd quart handy--cheap at the dollar stores.)

Run tap water into the jar; fill and dump out a few times to thoroughly rinse the filter.  Then, remove the filter and stand it on a dinner plate to dry.  When dry, roll it up and re-box it until you need it the next time!