Seen it dozens of times! It's VERY good! Based on the true story of the Brinks Robbery (the first million dollar robbery in American history I believe) that occured in Boston in 1950. (Though, like many movies they take some liberties with the true story. Doesn't matter, since it's a very entertaining movie)
Shortly before 7:30 p.m. on the evening of January 17, 1950, a group of armed,masked men emerged from 165 Prince Street in Boston, Massachusetts, dragging bags containing $1,218,211.29 in cash and $1,557,183.83 in checks, money orders,and other securities. These men had just committed the "crime of the century," the "perfect crime," the "fabulous Brink's robbery." At 7:27 p.m. as the robbers sped from the scene, a Brink's employee telephoned the Boston Police Department. Minutes later, police arrived at the Brink's building, and Special Agents of the FBI quickly joined in the investigation.
http://www.fbi.gov/libref/historic/famcases/brinks/brinks.htm