Not just another *uino clone, as this uses a 32-bit Atmel AT91SAM7X ARM microcontroller. Sure, it uses the same form factor (well, except for the breadboard-friendly mini version), but the Netduino uses the .NET micro framework.
What's that mean? Well, you're using a hugely more capable microcontroller (8-bit vs 32 bit), with much more memory (32kB vs 512kB), multi-thread programming, debugging, and event-based programming.
If you're comfortable with most MS programming tools, you'll be very comfortable trying out microcontrollers with the Netduino. And being Arduino shield-compatible, you've got a world of accessories available to you.
We're carrying the whole Netduino lineup, including the standard, the Plus (with ethernet & SD Card), and the 0.6"-wide DIP mini.