I have done several Flash Cache installations, and also have 1 512GB card on each controller in my lab environment with only SATA disks. (NFS, CIFs, and ISCSI)
In my lab I saw a boost in all protocols (and some NetApp features), and I also combined flexcache with my VMware NFS volumes. (the instructions to do this are in TR-3832)
But to answer your question, I just see a big boost in response times, and even a little write performance across the board.
These cards easily compensate for my slow SATA drives.
Here are some benchmarks for you,
Baseline submission: FAS3140 with 224 Fibre Channel drives
Second submission: FAS3140 with 56 Fibre Channel drives plus Flash Cache
Third submission: FAS3140 with 96 SATA drives plus Flash Cache