Product review (5 stars): Like This Type
I have used the accordion type of filters in our humidifiers for years.  I like the ease of replacement of the segments in the unit, and the amount of surface area for air to encounter.
One shipment of the type was not manufactured properly, in that the bonding strips on side of element to retain shape was missing on one side and element would not stay in the holder.  I have seen one, two and even three of these strips on replacement elements over the years, and not sure how that happens, but regardless of how many are on one side, there must be at least one on each side or they become a slinky.
Got replacements and are working fine - would highly recommend this type.