As someone who knew next to nothing about the character coming in to the film, I thought it was, well, OK. I certainly couldn't understand why people thought it was so terrible, but it didn't really make an impression on me either way. Oddly, I remember liking the Elektra spin-off much more.
I think most of the problem is just that people couldn't get rid of the Ben Affleck baggage. At the time, he and Matt Damon were these wunderkinds who had turned up and taken Hollywood by storm, but they still had that young pup image (it's hard to remember a time before Bourne when the idea of Matt Damon as an action hero was crazy). So people had real trouble taking Affleck seriously. But there was really nothing wrong with his performance and I agree with MovieBob's opinion that he could genuinely be a really good Batman.
I think most of the problem is just that people couldn't get rid of the Ben Affleck baggage. At the time, he and Matt Damon were these wunderkinds who had turned up and taken Hollywood by storm, but they still had that young pup image (it's hard to remember a time before Bourne when the idea of Matt Damon as an action hero was crazy). So people had real trouble taking Affleck seriously. But there was really nothing wrong with his performance and I agree with MovieBob's opinion that he could genuinely be a really good Batman.