I was looking for a fan that would really draw fresh air into my apartment (5 rooms, total ~1000sq ft).  It's very frustrating to me when it's nice and cool outside on a summer evening, but still sticky and stuffy inside from the heat of the day.  I've tried two of the little twin window fans, a larger oscillating fan on a stand, and small round fans in windows, but even all these of those in combination never really seemed to make a big difference in a reasonable amount of time.  I've been in houses with attic-mounted whole-house fans, and it's amazing how quickly they'll exchange the air in a whole house!  That's the effect I was hoping for.