Product review (4 stars): These are pretty good
The local hardware store carries a third-party product that fits the Aprilaire 45-series humidifers, and I think that one works better than the Aprilaire-branded one.  The third-party one I bought is all paper, and maybe it works a little better because it has more absorptive and evaporative surface without the metal?  I don't know.  They might not last as long as these, either.  But my goal was to get maximum humidity, and the Best Air ones did better at that job than the Aprilaire 45 did, so I knocked a star off this product.