Product review (5 stars): Perfect replacement filter
We just bought a new home that had an Aprilaire system...  A couple months of getting into the season here in Minnesota, checked on the existing filter and saw there was a decent amount of build-up so bought this one as a replacement/back-up.
Note that if you have a water softener, your filter will build-up quicker.  You can prolong the life of them by soaking in CLR. So we will use the two we have now to rotate out.