Is it actually even legal in the United States? From the Straight Dope:

Second, is it legal for the police to order me around and take my stuff? The experts agree: Probably. Many jurisdictions require civilians not only to surrender their property to a police officer in an emergency but to help officers subdue suspects on command.

How often does this happen? Policeman don't do it as frequently as the movies come out, I am guessing:

For example, snopes.com found an interview with one officer who had commandeered three vehicles in a 26-year career. Blackman v. City of Cincinnati, a 1942 Ohio Supreme Court case, involves a police officer who ordered a driver to use his vehicle to chase a fleeing felon.

Because this is an international site, I have included this:

Today most jurisdictions have rules empowering sheriffs, and the police generally, to command assistance from the public. These laws are sometimes called posse comitatus ("the power of the county") statutes. Ironically, England repealed its statute permitting sheriffs to command assistance in 1967. However, Blue says, "English constables are still thought to have the power to call upon bystanders to assist them in cases of reasonable necessity."

In America:

The Supreme Court has upheld the federal government's power to commandeer private property but imposed strict limits. In United States v. Russell, the court noted:

 

Extraordinary and unforeseen occasions arise, however, beyond all doubt, in cases of extreme necessity in time of war or of immediate and impending public danger, in which private property may be impressed into the public service, or may be seized and appropriated to the public use, or may even be destroyed without the consent of the owner . . . but the public danger must be immediate, imminent, and impending, and the emergency in the public service must be extreme and imperative, and such as will not admit of delay or a resort to any other source of supply, and the circumstances must be such as imperatively require the exercise of that extreme power in respect to the particular property so impressed, appropriated, or destroyed.

There you are... for Americans.

Source: The Straight Dope

Is it actually even legal in the United States? From the Straight Dope:

Second, is it legal for the police to order me around and take my stuff? The experts agree: Probably. Many jurisdictions require civilians not only to surrender their property to a police officer in an emergency but to help officers subdue suspects on command.

How often does this happen? Policeman don't do it as frequently as the movies come out, I am guessing:

For example, snopes.com found an interview with one officer who had commandeered three vehicles in a 26-year career. Blackman v. City of Cincinnati, a 1942 Ohio Supreme Court case, involves a police officer who ordered a driver to use his vehicle to chase a fleeing felon.

Because this is an international site, I have included this:

Today most jurisdictions have rules empowering sheriffs, and the police generally, to command assistance from the public. These laws are sometimes called posse comitatus ("the power of the county") statutes. Ironically, England repealed its statute permitting sheriffs to command assistance in 1967. However, Blue says, "English constables are still thought to have the power to call upon bystanders to assist them in cases of reasonable necessity."

In America:

The Supreme Court has upheld the federal government's power to commandeer private property but imposed strict limits. In United States v. Russell, the court noted:

Extraordinary and unforeseen occasions arise, however, beyond all doubt, in cases of extreme necessity in time of war or of immediate and impending public danger, in which private property may be impressed into the public service, or may be seized and appropriated to the public use, or may even be destroyed without the consent of the owner . . . but the public danger must be immediate, imminent, and impending, and the emergency in the public service must be extreme and imperative, and such as will not admit of delay or a resort to any other source of supply, and the circumstances must be such as imperatively require the exercise of that extreme power in respect to the particular property so impressed, appropriated, or destroyed.

There you are... for Americans.

Source: The Straight Dope