I just set up a StoreMI on my build 2 days ago. I have a 512GB HP EX920 NVMe as a boot drive (Asus Prime x470 Pro - love its heatsink for the NVMe drive) and a 4TB WD Red as storage. The Red was an 'upgrade' from a Seagate 2TB Constellation that was faster than the Red, but good God it was LOUD !! So in an effort to speed things up for the spinner, I added a Team Group L5 240GB SSD I picked up on the Egg for $30 as a 'tier' drive. Very easy to set up the software - just created a non-bootable pairing of the two drives, then the last part of the setup asked if I wanted to devote 2GB of system RAM to the setup as an aditional caching layer. I figured, ehhh, why not ? I don't push my system very hard at all, but I'd prefer to touch this setup for at least a few years.
Edit: added a run I just took to show what to expect. My drive went from a 108MB/Sec to 1800+. And that is exactly why I wanted to give this software a try ! But to be honest, benchmarks are one thing, a seat of the pants increase in system performance may not be noticeable. My boot NVMe reads at 3100MB/sec, but it doesn't boot into Windows any faster by my count.