The performance of Intel's Smart Response Technology (SRT) on Z68 Sandy Bridge motherboards has been examined by AnandTech:
it's not going to make your high end SSD obsolete. You'll still get
better overall performance by grabbing a large (80-160GB+) SSD,
putting your OS + applications on it, and manually moving all of your
large media files to a separate hard drive. What SRT does offer
however is a stepping stone to a full blown SSD + HDD setup and a
solution that doesn't require end user management.
In other words, by using SSD caching rather than separating the SSD and HDD, you trade a bit of performance consistency for convenience and simplicity for the user.