I was waiting for someone to bring up the RAM alternative so I could ask my stupid question: If it's 8Gb, why didn't they just use DDRx instead of flash?
Because Windows already takes care of this with SuperFetch. After you load the OS it immediately caches the applications you use most often into available ram, and removes them when you actually USE that ram. The cached applications load instantly, and the hard drive is none-the-wiser.
In fact, there used to be a grand push to put as much DDR cache in hard drives as was cost-effective, but the performance improvements have really disappeared after 32MB. The fact is that people access far too wide a range of large data files, and the OS has been smart enough to keep everything it needs memory-resident for awhile now.
The SSHD is an attempt to handle the two major weaknesses of SuperFetch:
(1) Boot time is still limited by the speed of your hard drive.
(2) Application loads are cached, but not data I/O. They are also removed from cache when memory is needed by other applications.
It does a surprisingly good job for such a small price premium. As long as you're not doing more server-oriented loads (i.e. streaming media to multiple users or serving large numbers of Bittorrent users) the non-volatile cache works well to compliment SuperFetch. It's a good compromise solution for people who don't want to pay the premium for an SSD + hard drive.