Dr. Louis P. Bucky, M.D., F.A.C.S., holds a bottle of Botox April 18, 2002 at the Plastic & Reconstructive Surgery Center at the Pennsylvania Hospital in Philadelphia - 

Activist investor William Ackman has set his eyes on a new target: Allergan.

Ackman has joined forces with Valeant Pharmaceuticals to purchase the Botox-maker for an undisclosed amount -- $50 billion is one educated guess. Ackman along with other so-called corporate raiders Carl Icahn and Nelson Peltz, have become famous, and sometimes infamous, for shaking up the companies they invest in.

Do they do more harm than good?

Bloomberg News reporter Tara Lachapelle joins Marketplace Morning Report host David Brancaccio to discuss the track record of activist investors, noting that more often than not, these in-it-to-win-it investors are actually a shareholder's friend.

Click on the audio player above to hear more. 

Follow David Brancaccio at @DavidBrancaccio