If you want to use your Raspberry Pi to drive motors to open doors, lift things, or power robotics; if you want to sense temperature and switch devices on and off; if you want to flash lights; if you want to teach it to play the glockenspiel; or if you want to learn about electronics from scratch, then Gertboard is for you. (This is the stuff I’m talking about when I blither on about physical computing here.) It comes with an assembly manual and a user manual (both of the above are direct download links to PDFs), which also act as a pair of teaching guides, bundled with plenty of programs to show you how to put things together.
The Gert board is way too expensive, $65 +$20 in shipping.
That was my initial impression, but it does have a bunch of drivers and additional peripherals in addition to the ATmega from an Arduino. Motor driver + A-D + D-A + mid-power transceivers + moderate power (relay/solenoid) drivers. It's an interesting demonstration of why the low price of RPI is misleading, though.
Also, IMNSHO, the whole "strapping area" concept is going to make it difficult for beginners to use.
I think what would be much more interesting and useful than a Gertboard is to use something like
Leonardo or Teensy along with something like Firmata so that the RPi can do all the high level work
and external communications and then let the Arduino handle all the real time i/o like PWM, i2c, servos,
A-D etc... as well as the 5v interface.
So then it becomes as simple as plugging in the Leonardo or Teensy into the USB port on the RPi.
This would make it much simpler/friendlier (and cheaper) for beginners.
Teensy would offer a great interface for breadboard experimenting
and Leonardo would offer access to using many of the existing Arduino shields.
It allows the RPi to be sealed up while the IO handled externally by the Arduino hardware
over USB.
Kidding aside, I totally agree with the "Comms" side, the RPi will be a great interface to a multitude of established Arduino units with whatever shields (or just Teensies or Minis with user HW).
A motor shield is ambitious at that price when DIY internet shelf price is nearly half is quite ridiculous. Cute glass fuse though