SWW doesn't seem to affect the overall crowd levels much, but they definitely make MGM very crowded on the weekends. I would say that the crowd levels in MGM during SWW are like mid-June; it is crowded, but not unmanagable if you use FastPass.
If you are a Star Wars fan, try the SWW at least once. If ot, then plan your MGM days for the middle of the week when it is less crowded.