I think they're only effective as a main/boot drive, as it can put your most commonly used files (mostly OS and programme files) on the SSD section of the drive. If you use it as a data drive, it will always be changing its mind as to what should be on the SSD section, so you're really just getting an expensive 5,400RPM data drive and you'd be better off with a normal 7,200RPM HDD (assuming you need a fast drive).
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(You don't choose which files go where, by the way, the drive does that automatically.)