Cached HDDs work fine for their INTENDED purpose - Speeding up boot time and a few MOST often used programs load time.
They do NOT improve the overall performance of the HDD, except as noted above. Non-cached file read and writes will be at the performance of the HDD mechanical limitations.
My preference is to use a dedicated SSD (preferably 102 or greater) as a OS Plus program drive and a separate HHD for storage