It does look well done. Care seems to have been taken to make pin assignments similar to Arduino to enable shield compatibility. That the pins are 3.3V outputs (tolerant to 5V inputs) means most shields should work. Plus the 60KB of RAM is a huge benefit for some projects (no EEPROM that I could see though).
The big gotcha (IMHO) is the reliance on the .NET development tools. It's not a simple switchover from the Arduino development environment to the Netduino one and you are essentially learning a new language and environment (though they are similar).
Still, given that it's a 32-bit processor running at 48 MHz with plenty of FLASH and RAM means it might make a nice "upgrade path" for people who find they have hit the wall with the Arduino.
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