The DriveStation DDR is an interesting implementation of hardware. Combining relatively cheap DDR memory with a hard disk could in theory fill the void between SSDs and conventional hard drives.
The actual result doesn't quite live up to that, and that's mainly due to how the software's been applied. When you use the TurboEX software to copy files it goes significantly faster, but when you access the drive directly (which most benchmarks do), it performs like a normal hard drive. This is because Buffalo did not implement the software very deeply in the operating system, where there is potential for more performance gains.
Seagate's solution with the SSHD is smarter that way. The SSD caching in the disk is not dependent upon the operating system, so more operations can be accelerated. The Buffalo DriveStation DDR is therefore interesting only if you are able to use the TurboEX software.
The DriveStation DDR 2 TB costs an average of £133 or € 165, and that's a lot more than a standard external 2TB disk. It's a lot to pay for faster transfer rates, but it works if you use it correctly.