I can't say I have a clue as to how successful they are, but they do make life a little easier. By using them it's possible to swap banner ads and products in and out seasonally, or in response to promotions.
However, banners don't work very well, and product images without title text don't work well either. If CJ could set it up so product images could be displayed WITH title text they'd be a lot more effective.
Sorry for the non-answer.