I just ordered one of the Alamode units.
As others have said, the Pi is great for some things, the Arduino for others. The alamode is nothing more than an Arduino that is able to be stacked on top of an Rpi via the GPIO pins without having to tie up a USB port to hook them together. You could do exactly the same thing with an Uno/Leo over USB.
That said, there are certainly projects that would benefit from the combination of the two platforms working together...
Pi - great for networking connectivity, GUI's, large memory/CPU processing (like maybe image processing) USB interfacing to other devices, etc.
Arduino - great for interfacing with sensors, particularly analog since it has A/D onboard, hardware PWM support, all the shields there are to interface with for things like Motor controllers, etc.
Marrying the two together should prove to be a very powerful platform!