How successful are publication agreement addenda?
I finally managed to take a bit of time today to research this question further, so here goes:

First, there hasn't been much data compiled on the topic. Science Commons's FAQ on publication agreement addenda says:


  Anecdotal evidence from a variety of sources reveals that many scholarly authors have had success over the years in altering the terms of a publication agreement. Sometimes this has been done by marking up the publisher’s agreement; other times the change has been done through use of an addendum. However, we are not aware of any statistical data concerning the success rate.


while the more recent Creative Commons Science page fails to mention success statistics at all. 

However, we can measure the “success” of such addenda (or its influence) by looking at the reactions of various publishers to it. The MIT Libraries actually maintain a list of publishers and their stance. In a similar vein, the useful (and by now, well-known) SHERPA/RoMEO online database maintains a publishers statistics:

                  

If you look at these lists of publishers/journals, and you cross that information with the hottest and most common journals in each field, you see that things are very field-dependent:


In mathematics, the status of publications is quite different from the other hard sciences listed below. I do not feel competent enough to comment on it, but it seems to be working okay, with most mathematicians able to publish preprints or postprints on their personal websites, or to publish outside the “conventional model” (read: arXiv). This summary of various publishers' policies in the field is quite good (and recent).
Physics: due to the success of arXiv and the open-friendly stance of physics publishers (both American Institute of Physics and American Physical Society), physics seems to be one of the favorable fields for open publishing.
Traditional biology journals tend to belong to the big-money editors, who don't favor open publishing. However, open journals seem to have made good progress in this field in the last few years.
Chemistry is simply a nightmare. Most quality journals in chemistry are published by the American Chemical Society, the Royal Society of Chemistry, Wiley and Elsevier. All four of them have taken a hard stance against the open publishing movement. Open access journals have very little (read: absolutely no) success breaking through in the field of chemistry :(


Above are the fields I felt comfortable enough discussing. I welcome edits (or comments) that add information on other fields (including humanities)!



So, all in all, statistics pertaining to publication agreement addenda specifically are hard to find (if they exist). But, we can measure the success of the open publication model, and it  varies widely between fields.