I agree the marketing was terrible and misleading.
However, regardless, the film managed to do very well when it came out. Some can argue it is/was just the appeal of Aniston and or Vince Vanugh... But I think once people see the film they realize how good a film it is and how talented both of them are in their own right, their personal lives (of each) aside.
This film is very similar to "Prime" with Uma Thurman and Bryan Greenburg where it addresses a side of relationships (in Prime's case, May/December romances) most people don't acknowledge -- at least, Hollywood doesn't -- But it does it with a real sense of humor and a sense or realism that doesn't talk down to the audience and treats them like adults. This is what I think The Break-up's strengths are and why it is more like an independent film more than a mainstream Hollywood film. Relationships can be both exciting and frustrating and the Break-up captures both of those extremes simultaneously AND it also manages to manipulate your point of view of the characters and what happens to them if you are male or female.
Men will argue Brooke was too demanding and your typical woman who expects the guy to read her mind while women will tend to side with her and view Gary as an insensitive jerk who never really made the effort to change, or be more sensitive to Brooke's needs. When it works on both levels like this you know the film is treating the audience like adults because this is what adult relationships are really like; there is no black-and-white, good-guy-or-bad-guy. It is "what it is" and sometimes you don't even know how it got that way like Gary and Brooke realize in the end, but it is too late.