Here below is some of my SSHD experience.
 
I have used an SSHD as a boot drive for many years in both a desktop (Mac Pro and MacBook Pro; OS X and Windows Bootcamp). The SSDH is beneficial for reducing boot times and for loading programs. As correctly stated in other posts, the 8GB SSD portion of the SSHD 'learns' frequently read files and promotes them to the SSD cache function. 
 
After migrating those same machines to SSD's, the uplift in speed from SSHD to SSD is evident. As usual, with us humans, the thrill is short lived, and we immediately adapt to the new tempo of events.  
 
I had built RAID-0 arrays with SSD's for the ultimate in throughput. My experience with SONAR projects in a 1700 Mbytes/sec (!) RAID-0 array was that the biggest benefit was that Auto-Save never gets in the way even with huge projects. Other than that, tracking, editing, mixing, and other 'normal' activities didn't change my user experience very much. I use a separate dedicated machine for soft synths, so I can't comment on any benefits the RAID-0 array may bring to that angle of usage. Since my real intent for using the RAID-0 array is video editing, I'm not disappointed with the almost neutral audio performance differences; but this should serve as a cautionary tale for those who are seeking computer thrills through SSD implementation. 
 
By the way, I had intended to try an SSHD RAID-0 array, but learned that lower-grade arrays are used in the SSD cache, and they are not able to saturate the SATA interface in read bursts as SSD's are able to do.  If you have the stomach for it, you can read the whole sordid tale in the TECH section of my website ... www.tedlandstudio.com