It works very well in terms of keeping SSD speed/performance and the storage size of the platter HDD.

This was a year before the fusion drive was supported by Apple so I put the OS/Apps on the SSD and the User folder on the HDD.

In my case, I went with the 2 HDD setup in my Early 2011 MBP instead of Fusion (mainly because Logical Volumes were not yet supported when I did my mod) put in an SSD in the main HD bay, and put the platter drive in the optical bay with the harddrive caddy. With the 2 HDD setup, you reduce the risk of losing all your data at once, while still retaining a large portion of the speed/storage benefits of Fusion, but with more flexibility. Often, these files far surpass the 4GB size, so you will see fast SSD transfer speeds followed by a precipitous drop in speed when it transfers over to the hard drive. The trouble comes when you start working with larger files, such as with pro audio, video and large-scale photo work. Once writing is complete, data will be moved from the SSD to the hard drive until there is 4GB free on the SSD again. A Fusion drive will first fill the faster SSD portion, then start filling the slower hard drive.

Used 2012 macbook pro 3 tb fusion drive