Should Actors Direct Themselves in Movies?

Helen Hunt has done it. Woody Allen has done it. Clint Eastwood has done it. Now Ben Affleck is joining the ever-growing group of actors who star in movies they direct.

It seems like it might be tough to maintain a "big picture" view of a project if the director is also the star of the film, but then again, he/she knows better than anyone else precisely how the performance should look.

What do you think? Is it a good idea for actors to direct themselves?