So far having solid state cache for a hard drive is an idea which looks great on a paper, but practically everything that has been offered shows the performance - and we're talking about the real workload and the real user experiences - closer to the hard drive than to the solid state device. IMHO, since, apparently, we have a fairly large number of cache misses or some other anomalies, having the solid state cache which is 1000 faster than the traditional NAN-based one won't make too much difference.
On the other hand, having the solid state device which only 10 times slower than DDR would make it excellent virtual storage. you can put 64GB of DDR4 on your server and then get 350GB slab of Optane. For all practical purposes you have 350GB of main memory. Swapping the working sets in and out would happen, for all practical purposes, instantly. But of course that's solution for data center, not for the regular user.