August 18, 2012:
Six years ago, the U.S. Air Force declassified details about a secret program that wasn't much of a secret. "Constant Peg" was the 1977-1988 program that put dozens of Soviet warplanes into American service for training and evaluation. In 1985, for example, there were 36 MiG-21s and MiG-23s. There had been some MiG-17s used for over a decade, but by the early 80s they were phased out. The experience gained from this heavy use of MiG aircraft continues to benefit the U.S. Air Force and Navy. Russian warplanes continue to be widely used by potential American opponents, and those aircraft contain design and operational characteristics that have persisted for decades.
During the 1980s these Russian aircraft were mainly used for training U.S. pilots and, more importantly, finding out what kind of air force the Soviet Union had, and what it could do. A lot of what was learned leaked out over the years. But in the six years since Constant Peg came out of the cold, there has been quite a lot of chatter among pilots in air forces that operate older Soviet aircraft, or might be facing them. In Taiwan, for example, there was relief that it was now public knowledge about how most Chinese aircraft (which are MiG-21 clones) were, well, pretty third rate. While the MiG-21 and MiG-23 were capable of some fancy flying, it took a skilled and experienced pilot to do this. The Russians had few such pilots. For less skilled pilots, these aircraft were dangerous to operate. But for American fighter pilots, seeing these MiGs in the air, handled by experienced pilots, it was a solid learning experience. The Russian fighters, painted as they would be in Russian service, showed off their diminutive size (compared to Western warplanes) and nimble handling (when flown by a skilled pilot) often won those training encounters. The trainees learned that they were facing a "worst case" situation, and were taught how to exploit the weaknesses of Russian aircraft.
While the Constant Peg pilots leaned that the Soviet aircraft were quite maneuverable, they also learned about a lot of interesting quirks, which could be exploited in combat. Perhaps more important was what American aircraft maintainers learned about how well Soviet aircraft held up. Not well, it turned out. Based on the American experience with these Soviet aircraft, it was discovered that the rumors of much higher accident rates in the Soviet air forces were true. Soviet accident rates turned out to be more than five times higher than American ones. Maintaining these Soviet aircraft day after day revealed why most users of these aircraft didn't get much time in the air. The aircraft were poorly built, with often shoddy components. Even if the Soviets had wanted to give their pilots more air time (and make them more effective), it was not possible. Soviet aircraft simply could not be flown frequently enough. All this was in support of Soviet military doctrine, which called for the maximum number of aircraft to be ready for the early stages of a war. The concept was that quantity would overwhelm quantity. This never worked in practice, and even the Soviets were having doubts at the end of the Cold War.
The Russians were adapting to this reality with the introduction of the MiG-29 and Su-27 in the 1980s. These were designed and built to Western standards and were meant to be used for a lot of training flights. While this was too late to save the Soviet Union (which went bankrupt and dissolved in 1991), those two aircraft were exported heavily to China and India, saving their manufacturers from bankruptcy and keeping Russian military aviation alive. The U.S. later got to closely examine, and operate, many MiG-29s that were operated by East European nations that were, until 1989, unwilling Russian allies.
The U.S. Air Force didn't want to run their original Soviet aircraft collection into the ground, so they carefully collected data on flight characteristics of the MiGs, and found ways to use American aircraft, flown by pilots, who could fly it like a MiG. But that did not make up for the visual experience. It was startling to see the MiGs in the sky, painted in realistic colors (including the red stars). Thousands of American pilots did get this experience, and passed on to others in their units. These days, Western aircraft, like F-5s are used to simulate enemy (usually Russian designed) aircraft. The "aggressor" pilots are trained to make their aircraft operate like various types of Russian and Chinese warplanes.
The source of the Constant Peg MiGs is still a secret, apparently to spare the providing nations embarrassment. However, word on the street is that most of the MiGs were purchased from Middle Eastern countries who, at one time or another, were big users of Soviet aircraft. Egypt is often mentioned, but there were others as well.
The MiGs were based at a secret facility near Las Vegas, and were part of the 4477th Test and Evaluation Squadron. The MiGs ended up flying over 15,000 sorties, far more than they would have if operated by a regular Soviet squadron.

http://www.strategypage.com/htmw/htmurph/articles/20120818.aspx