Some possible flaws in the details of Shawyer's calculations that have been suggested include:
The analysis of the effect of the wave reflecting off of the two ends of the waveguide may be incomplete due to failing to consider the forces caused by the wave impinging upon the conical side of the waveguide; if the large end of the waveguide and the combined system of the small end and the conical sides are considered separately, they both have the same cross-sectional area so one would expect that the radiation pressure upon them would be equal in magnitude yet opposite in direction, thus producing zero net force
The New Scientist article also mentions that as the EMDrive begins to move, the microwave energy resonating in the chamber is drained very quickly - quicker than it can be replenished. This implies that the system is suitable for producing a static force, but not acceleration. This means the EMDrive, if developed further, could be suitable for hovering vehicles and objects, but not for flying around as people would like to imagine. Think of it as a frictionless replacement for wheels. With this consideration, the EMDrive no more violates conservation of momentum and energy than, say, a magnetically levitating train. Instead, it violates the general principle of relativity since it would be possible to distinguish when the device is in freefall (device works) and in free space (device does not work, runs out of energy). This is based on the New Scientist article. However it is possible it will run fine, in pulses, in both zero-gravity and freefall.
The consideration of different frames of inertial reference may be invalid. In other words, the operation of the device may boil down to construction of a ship with a large sail at one end and a small sail at the other end, then standing in the middle and blowing uniformly in all directions; this would only create forces within the boat rather than a force capable of propelling the boat away from any fixed body.
My opinion: It seems others have been willing to give him some more money to build something which he claims will provide a lot of force. The experimental effort looks like it will clearly settle the issue of whether it works. Plus the previous prototype work could be replicated and the existing prototypes analyzed.
There is more discussion at the wikipedia discussion area