The first results speak for themselves. HDDs love optane cache.
This is literally true. In some benchmarks, Optane Memory works best paired with HDD, its worse paired with SSD and SSHD. Probably that's why Intel optimized for?
SSDs with Optane won't bring noticeable benefits. Diminishing returns. It's really about using with HDD. I have i3 7100, 16GB Optane Memory, 2TB Seagate 7200RPM.
The hard drive spins way less than it used to.
Special scenario that saw a big benefit: There was a flash game I played in firefox that used 90% CPU. It loaded slow, and the game experience was laggy. Multi-tasking became hard. It would lag browsing. I remembered on my SSD laptop Firefox nor Chrome had such issues. Chrome is faster on my i3 7100 system, but it has compatibility issues.
With Optane Memory the CPU usage went down to 30-40% or so and its very playable. So that 90% CPU utilization consisted of idle threads. Multi-tasking is possible.