It will fit on any Arduino (including Mega) and allows for an additional shield to be added on top (shield will be connected to the base Arduino) other pins will be free.
It can be powered via the Base Arduino or the VIN pins - power will be routed through the VIN pin from base Arduino and back out as +5v to Core2/3duino.
Why do this? well, the short answer is - why not? You end up with:
3 separate processors
60 input/output of which:
24 digital I/O pins
18 pwm pins
18 analog inputs
including 6 external interrupts
...all for about $25 more than your Arduino. (about $10-12 each)
Anyways, it only took about an hour to modify the Core2duino Eagle file into this.
This is a fun project to try and the .brd file has adequate spacing for ease with etching.
Most of these instructions are very similar to the Core2duino... differences are noted.
I will be happy to help if anyone has questions.
The next version is likely to have an FTDI programming cable connection for easy in system programming.