Their Arduino shield can be used to replace the NXT brick and is capable of interfacing with 4 NXT sensor and 2 NXT motors.
So is it better than the NXT brick itself? AS John points out "Yes and No"
So the shield and Arduino beat out Lego’s microcontroller, right? Well, yes and no. In our explorations we found ourselves very impressed with the NXT brick, which is the default Mindstorms microcontroller. It’s so good that many hackers have figured out how to put alternate operating systems on the NXT brick like LeJos and NXT-C. Then there is the fact that the NXT can control 3 motors and our shield only two. That said, there is a lot to like about the Arduino approach. Simply being able to use all of Arduino’s vast ecosystem of hardware and code will do wonders for your robot. Wayne and Layne are also developing a motor controller that can run five motors and has a built-in XBee connector. It’s not ready to buy or build yet but you can see it in action here.More info here - http://blog.makezine.com/2013/02/04/the-monday-jolt-bricktronics-expands-the-possibilities-of-lego/