The ACLU(link available soon here):

“Any verdict resulting from such a flawed system is a betrayal of American values. The rules for the Guantánamo military commissions are so flawed that justice could never be served. From start to finish, this has been a monumental debacle of American justice. The judgment against Hamdan undoubtedly will be challenged in legitimate courts, but there is no appeal from the judgment of future generations. This system was devised to permit the prosecution of alleged wrongdoing by detainees, while continuing to cover up the wrongdoing by government interrogators. Trials that are shrouded in secrecy and tainted by coercion are the very antithesis of American justice.”

...“In the strange world of Guantánamo justice, even if Hamdan had been acquitted on all charges, he would have been detained indefinitely. Nowhere else in the U.S. justice system can someone be held for life regardless of whether he is convicted or acquitted of a crime. Today’s outcome represents nothing more than an illusion of justice. It is time to shut down these commissions and put an end to this shameful chapter in American history.”

Update: The Center for Constitutional Rights:

“Hamdan’s trial violated two of the most fundamental criminal justice principles accepted by all civilized nations: the prohibition on the use of coerced evidence and the prohibition on retroactive criminal laws.

The decision to keep these cases out of the ordinary criminal courts will produce years of appeals over novel legal issues raised by the untested military commissions system. Even after those appeals are finished, the process will never be seen as legitimate by the world. This case was the first trial run of the commissions system, and the decision proves nothing except that the system itself should be scrapped. Terrorism-related crimes should be tried in the time-tested domestic criminal justice system, a system whose rules have been designed over the centuries with two goals: to seek out the truth and secure justice.”