Hot Hardware published a review on the Intel Optane Memory With 3D XPoint Technology Caches Slower Drives For A Performance Boost A quote from the article:

Early last week, Intel revealed its Optane SSD DC P4800X featuring 3D Xpoint memory technology. The Optane SSD DC P4800X is an enterprise-class product, targeted at data centers and mission-critical applications where reliable, ultra-low-latency access to large amounts of data is paramount. Today, Intel is introducing another new product featuring 3D Xpoint memory, but this is one quite literally at the other end of the spectrum. The Intel Optane Memory devices we'll be discussing here looks like other current generation 80mm M.2 ?gumstick? type solid state drives. However, Intel Optane Memory is meant to be used as an accelerator for systems featuring relatively low-speed storage devices, like hard drives...

 Intel Optane Memory With 3D XPoint Technology Caches Slower Drives For A Performance Boost @ HotHardware