They've implemented Optane caches in the large HDD arrays on the servers where I work.
Access times and general throughputs are definitely faster and more responsive than they used to be. But also definitely not as fast as pure SSD performance. In this application it's an excellent bang-for-the-buck solution, more raw storage available with "acceptable" performance at a given price. In a consumer PC it would be a waste because SSDs offer better performance for the same cost, unless you plan to install and demand some performance from huge HDD arrays.
Optane does not speed up decent SSD performances and it does increase performances of any RAIDs made with such SSDs. Intel marketing has vastly overhyped this technology, it does have a strong niche in enterprise environments, but it's utterly useless to consumers.
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