[nomel]

I've worked with the larger version of this in the past (Beaglebone Black). I really think the two 2 200MHz PRUs [1] are very neat, allowing some nice realtime applications with a standard *nix distribution. They can run without the main CPU, and even wake the main CPU, for lower power tasks. You can access the PRU memory directly with mmap, and the PRU access to system memory space.

I used them to bit-bang MDIO (used by ethernet PHYs) and JTAG for debugging and burning fuses (strict timing requirements) in an IC.

It's cheap, easy to use, with good support, and much less of a barrier than an FPGA+ARM solution.

[1] http://elinux.org/images/d/da/Am335xPruReferenceGuide.pdf (page 14 for block diagram)