A New Arduino (or Two)

Arduino have announced a couple of new boards, still based on the same Atmel family of controllers. The Arduino UNO and Arduino Mega 2560 are successors to the current Duemilanove and Mega. USB has moved from the ubiquitous FTDI chip to a (presumably cheaper) ATMega8U2.

Now, the Arduino has achieved huge popularity, partly down to its "open" architecture. But there are many many open design boards out there, and have been for years, I wonder how and why Arduino really became "viral" because there are many better processors and dev boards out there.