we have 3 room A/C's (bedroom, living room, computer room), and all 3 have been getting a serious workout this summer...
this is our 10th year in the house, and i'd say that 4 or 5 of the summers were cool enough that all we needed were a few strategically placed fans...
its an old house that's slow to cool down or warm up when the weather shifts, which it does on a regular basis, so it might be 50 degrees outside, but it still feels like its 90 outside in the house