Scramjets: Viable Hypersonic Solution or Expensive Pipe Dream?

In recent years, enormous resources have been allocated to the research and development of supersonic ramjet engines called scramjets. These engines have no moving parts, do not need to carry a heavy tank of oxidizer (as rockets do), and are theoretically capable of speeds up to Mach 25.
The basic principle of the scramjet is similar to that of a standard ramjet engine. Air from the atmosphere enters the engine at the inlet, where it is compressed to high pressure and temperature. Fuel is added to achieve combustion. The combusted air is then directed out of the exit nozzle to provide thrust.
The key difference between a ramjet and a scramjet is that in the scramjet, the airflow through the engine remains supersonic throughout the process, as opposed to slowing below Mach 1 in the ramjet design. Keeping the flow supersonic avoids a large shock caused by slowing the flow to subsonic speeds. This results in a much lower drag on the engine. Also, since the gas is slowed less by the scramjet, it is heated less, which results in a more efficient combustion.
One of the major drawbacks of scramjets is that in order for flow to remain supersonic through the entire engine, the scramjet must be moving at a speed of about Mach 5 or greater for proper functioning. Therefore, the scramjet must reach Mach 5 by conventional means - which is no small task. Furthermore, not much is known about the fluid dynamics involved in the hypersonic regime. Computational fluid dynamics (CFD) is just getting to the point where it can begin to work on some of the problems of hypersonic flight. Some of these problems include turbulent mixing, two-phase flow, and flow separation.
Today major research is being done by groups such as NASA and the U.S. Navy. Research is also being done at the academic level. NASA and aerospace firms such as ATK have teamed up with a number of schools in Virginia, including Virginia Tech and the University of Virginia, to form the Hy-V program (pronounced high five). The goal of this project is to test how well wind-tunnel scramjet tests reflect actual scramjet flight.
Scramjet technology may one day provide civilians with extremely fast transcontinental flight, space programs with single-stage-to-orbit flight, or the military with high-speed reconnaissance aircraft like the conventionally powered SR-71 Blackbird. This is, however, a very new and untested technology that many suspect to be too good to be true. Only time will tell.