Updated:  

Still, Homeland Security officials praised the aircraft as a safe and important tool that over land has contributed to the seizing of more than 22,000 pounds of marijuana and the apprehension of 5,000 illegal immigrants. ...

The program has its critics. The union for Border Patrol agents has criticized the drones as costly and inefficient and has suggested the money would be better spent on adding workers and equipment on the ground.http://www.nytimes.com/2009/12/08/us/08drone.html

By Randal C. Archibold - The New York Times

Illegal Immigration
Attrition through Enforcement
border control