Product review (3 stars): Does the job but not the right size
This filter is hard to find, as the other reviewers state. It does not come ring shaped. Rather, it is shipped flat, with a couple of folds. You have to reshape it to fit in the humidifier.
Also, it is a tad small. You cannot form a complete ring with it. That probably reduces the dust reduction factor of the humidifier, which is one of advertised factors in the listing ("virtually eliminates dust").

However, since the original filter that came with the humidifier was a perfect ring of the perfect size, the customer is being short changed with this product. There seems to be no alternative to it, however.

Update: I recently (Mar 24) purchased two MAF-2 Essick Filters at homedepot.com less expensive and exactly the right shape and size. Fit easily into the humidifier just like the original one. So, I would say, get this only if the homedepot one isn't available.