I’m just starting to get my head around the mapping possibilities of Google Fusion Tables. The main advantage over KMLayer, as far as I can see, is that very large and complex maps can be downloaded much faster. How exactly, I’m not sure. I uploaded a 17MB KML file this week and the resulting map loaded acceptably. There much be something in the background loading only the information needed at any given view level. More to come this week.
The Guardian published a FusionTables-based map this week which shows every census tract in England (32,482 polygons). It loads just fine, despite being somewhat more complex than a poll-level election map of Ontario election results would be.