SATA Express is an interface to either connect a hard drive to PCIe lanes, or up to two drives via SATA. Obviously, PCIe bandwidth over a cable connection is the real draw. To use the full speed, however, the drive needs to be able to communicate over PCIe. Currently, the standard uses two PCI Express 2.0 lanes (1 GB/s).
Now that Z97 and H97 have launched, WD is set to show off the technology at Computex. The above image is apparently of a dual-drive product, containing 4TB of rotating media and 128GB of SSD memory. I am immediately reminded of the Western Digital Black2 dual drive which Allyn reviewed last November. That product crammed a 120GB SSD into a 2.5" 1TB HDD, which appeared to the system as two separate drives. The drive has "Technology Demonstration" written in red font right on it, but it could be a good representation of what the company is thinking about.
WD also asserts that their prototype uses standard AHCI drivers, for OS compatibility.
If you want to see this product in action, then — well — you kind-of need to be at Computex. At some point, you might be able to see it in your own PC. When? How much? No pricing and availability, again, because it is a tech demo.