My current forums are brand new, so not much to show there. In the past it has helped. Although it doesn't quite need to be post exchanges, just any artificial traffic to drive organic traffic into registering is a good thing to get a community going. The effects are also dependent on the current community, great for brand new sites to get going or small communities that need a boost, but large and active communities will see little outcome from the work.
However if you are talking about the actual people that you do the post exchanges with sticking around? Then no, very little success there, but that is not really the goal and is just a perk if it happens.
You have to remember, that most people doing exchanges are current forum owners already well invested into their own communities and likely a series of forums outside of their own that they use. It is one of the hardest groups to pull into a new forum.