Bought the 64GB (4X16GB) kit to max out the memory in my new iMac19,1 which came out in March. Definitely noticed a difference when editing large files as the 8GB (2X4GB) it came standard with was slowing the computer down. This is the best 2019 iMac you can buy off the shelf at the Apple Store. It is HALF the cost Apple would charge for the memory alone!
Unfortunately, I chose the 2TB Fusion drive as I had no choice. While it is still reasonably fast with 128GB of SSD built in, startups and load times lag compared to a iMac 18,3 with the Apple 512GB SSD option.
Can't wait for OWC to release a DIY SSD upgrade kit for the iMac 19X models! I will buy one as soon as they do.