The Mcell is an interesting beast. In terms of size, it fits in the standard 3.5" form factor, but inside it actually contains a 2.5" 5400rpm hard drive, a stick of 1GB DDR2 RAM, and a special chip from DTS that contains a real-time OS and CPU. Don't let the slow RPM fool you. The combination of the write-through cache and the 2.5" disc should afford much quicker access times than with 3.5" 7200rpm drives.
According to DTS, the drive sees speeds above 110MBps in random read tests for data sizes of 64KB to 512MB. Even though performance will almost certainly drop when using data samples larger that the cache size, the numbers are still impressive as it apparently provides faster access times than many 7,200rpm SATA drives available today. Mcell is currently only available in Japan in 80GB, 120GB, and 160GB sizes, with the 80GB model selling for about $136.