Most of us are familiar with the Arduino ... pin layout (Arduino shields likely won’t fit) and more I/Os than the ATmegas have. Sure, it’s not as practical as an actual Arduino Uno, but ...

It’s a significant update. The Arduino UNO and Arduino Mega 2560 update the Duemilanove and Mega respectively. They now use an ATMega8U2 instead of an FTDI USB to serial converter chip.