Newb question: Can this drive be used as the primary mass storage in a desktop build?
Your question is a little ambiguous. Using the term "primary" usually means your OS drive (your C: drive). On the flip side, "mass storage" typically refers to a different drive (like a D: drive). I think you mean the latter (the D: drive). If that's the case, I'm not sure a hybrid is a good idea.
The benefit of the hybrid is (over time) it "learns" what you access the most. That information is stored on the SSD part while the lesser accessed stuff is stored on the standard HDD. For an OS drive (or for a system with just one drive), this is a decent option - the OS would reside on the SSD part (with some frequently used applications) with data on the HDD. As a mass storage device, I think you would be accessing different data all the time which means your data may (or may not) be on the SSD, thus limiting the benefit of the hybrid drive.
Then again, if you're just asking if it's even feasible to use a hybrid drive however you're intending, my suggestion would be to see if you can find a manual online that you could read - if there are special instructions on how to set up the drive, it should be listed there. Next, check some reviews online and see if anyone mentions the ease (or difficulty) of setting up the drive.
Hope this helps.