Affleck isn't a bad actor at all. He has made some awful, and some unfortunate, film choices though; unfortunately people put the failure of those pictures firmly on his shoulders, as though his very presence is the thing that drags down the quality of an otherwise excellent film, which isn't the case. It's just that they're shit films.
Thought he did a good job in the Kevin Smith films and Good Will Hunting. Then there's the fact that, as you say, he wrote some of the latter film. Although, it should probably be noted that most of the work was done by Matt Damon, who if I remember correctly, wrote the beginnings of the screenplay for his final piece in college, and later completed it with the help of Affleck. It's hard to say how much Affleck was responsible for though, Damon could have just included him in the credits as he's a close friend. It's a fantastic screenplay though.
Only directed one film, Gone Baby Gone (starring his little brother, who's fantastic in his own right), and that was impressive.