This is much smaller than most Arduinos (certainly smaller than the official Arduinos).

Processor-wise I guess it should be comparable to the Arduino Due (http://arduino.cc/en/Main/ArduinoBoardDue) which has an 84 MHz Cortex-M3 (the Spark Core runs at 72 MHz). The Core seems to have more code space, with a 2 MB external flash.

It also looks like it will be well-integrated and I think that's where the real value is.

There's an incredible amount of value in "well-integrated". I wasted weeks trying to get an Arduino to have enough control over an Xbee to get it to even see wifi networks. And Xbees were designed to work over existing wifi frequencies...

Uploading sketches via wifi is a unique feature, isn't it?