To date, only small parts of the proposed NMD system have been tested. This is a complicated system and is being tested incrementally. You develop a component, test it, find the problems, test until you're satisfied, then move on to the next component.

Step back from NMD for a moment and consider something more familiar - a new airliner. Long before the first flight, the engines will be tested for hundreds of hours in test stands. So too will all of the other aircraft systems - the hydraulics, the structure, pressurization system, fuel systems, landing gear, etc. Thousands of hours of testing will take place before the plane makes its first flight. Even then, only the crew gets to ride during the test flights. These test flights will use several airframes and hundreds of flight hours before the plane is allowed to enter service.

The same kind of thing is happening with NMD. The key component you've been hearing about is the kinetic kill interceptor vehicle. This vehicle has been in development for many years, long before the flight tests began. Those flight 4 tests were done using a smaller rocket. The larger rocket planned for the operational system just had its first flight test a few weeks ago. This rocket has a reported range of about 3000 miles compared to just a few hundred miles for the test vehicle. Future tests will add an interceptor vehicle to this new rocket to try for intercepts at far greater ranges than anything attempted to date. However, there are other components to the NMD system besides rockets and interceptor vehicles. First, there's a new constellation of launch detection and tracking satellites called SBIR (Space Based Infrared). These systems will be much more capable than today's DSP system. In addition, a new X-band radar system will be built to provide very accurate mid-course tracking of incoming targets. All of these components make up NMD and each will require testing. There is still a lot of work to do before anyone will even begin to consider this system operational. In all honesty, the testing has barely began, so it's way to premature to say the system does or does not work.

http://www.fas.org/spp/military/program/warning/sbir.htm
http://www.acq.osd.mil/bmdo/bmdolink/html/nmd.html