Is there any role for Congressional oversight of Executive Branch or not?

http://blogs.usatoday.com/ondeadline/2008/07/rove-ignores-su.html
 
Rove ignores subpoena to testify about Justice Department politicization
Mr. Rove will respectfully decline to appear before the Subcommittee on July 10 on the grounds that Executive Privilege confers upon him immunity 
 
A few minutes ago, the Associated Press reported that Rep. Linda Sanchez, the California Democrat who chairs the subcommittee, ruled that Rove's claim was invalid. 
 
http://www.usatoday.com/news/washington/executive/2008-07-10-rove-subpoena_N.htm
Rove defies subpoena from Hill Dems on Justice Dept. 
Updated 2d 18h ago 
 
Lawmakers subpoenaed Rove in May in an effort to force him to talk about whether he played a role in prosecutors' decisions to pursue cases against Democrats, such as former Alabama Gov. Don Siegelman, or in firing federal prosecutors considered disloyal to the Bush administration.
 
Rep. Linda Sanchez, chairman of a House subcommittee, ruled with backing from fellow Democrats on the panel that Rove was breaking the law by refusing to cooperate  perhaps the first step toward holding him in contempt of Congress.