Activist investor Carl Icahn (L) speaks during a media conference at the St. Regis in New York City.  - 

It’s been a newsy week for activist investors. William Ackman resigned from J.C. Penney’s board and Carl Icahn took a big stake in Apple, which saw its share price rise after Icahn sent a Twitter message about the company.

Activist investors can offer a bad-boy appeal -- they buy up shares, flex their muscles, and try to force changes that increase shareholder value. 

I think the most successful thing they do is forcing boards to reexamine dogma,” says Charles Elson of the Weinberg Center for Corporate Governance.

He cites Home Depot, Waste Management, and AutoZone as companies that have benefited from activist investors, as well as Yahoo, where a shareholder led the charge to install Marissa Mayer, the company’s sixth CEO in five rocky years.

But all the stories are not sucessful ones, according to Wharton professor Mike Useem. Take J.C. Penney. There, Ackman pushed for a new CEO, but the company suffered steep losses when a new strategy failed.  

If an activist investor ultimately brings in the wrong CEO, we can see dramatic swings in the performance of the company so involved,” says Useem.

J.C. Penney is now trying to recover, but without Ackman on its board.