Product review (2 stars): Marginal Performance at the Price, Poor Support from Whirlpool
I have purchased a few of these filters before and they seemed to work ok, though price continues to go up.  I recently installed one from a two pack that I bought on Amazon and the flow was very poor.  When I tested the water coming for total dissolved solids (TDS), it measured in the 180's, though the tap water was only in the 150's.  I replaced this filter with the second filter and things got better, though TDS was still in the 90's.  I called the toll free number on the packing to see about getting a replacement and they (Whirlpool) said that I had to return to Amazon, since I bought it from them.  When I pressed them about the return, they insinuated that Amazon is not actually authorized to sell their products (I have a hard time believing that) and if I want to have my products warranted, I had to buy from them. Unfortunately for me, I had bought these for several months in advance of needing them, so my return window with Amazon was closed.

Bottom line on this filter:  There are likely others that work as well or better for less, but if you decide to buy them, buy them one at a time and install them before your Amazon return window closes.  Hope this helps!