Great topic. One of the things people rarely think about when their rankings stay the same but traffic falls way off, is whether the targeted query phrase changed position in the Suggestions drop-down. That can explain a lot of mysteries!
One big factor in Google Suggest is query volume. People have tried to influence Suggest by generating artificial search traffic (MTurk and the like) and it used to work pretty easily. These days from the reports I hear, not so much.
Another factor that needs to be taken into account is that Suggestions are heavily geo-located and can vary even within the same city. That creates some serious analysis challenges!