TomsHardware performed a thorough review of a Seagate Momentus XT Hybrid Hard Drives. I recommend reading it.
"Really, the only time Seagate's Momentus XT slowed down drastically compared to an SSD was when we installed the operating system and applications. Once everything was fully loaded, however, performance rapidly improved as the drive's software algorithms pulled the most frequently-access data into flash, bestowing very SSD-like qualities to it. At that point, it was frankly hard to tell the difference during most common tasks. "
The radar plot at the end shows the mix of compromises, and the performance suggests it's worth a look if you're otherwise buying large capacity.