I have central air, and I would never want to live in my area without it again. But, we have weeks on end of 100+ degree heat, no real cool down at night (some nights stay in the high 80s), and it's miserable. We also have cold winters, snow, ice, etc, so the furnace with the same air ducts is used all winter as well.
However, if it was only needed a few days or weeks a year, I wouldn't bother. We have a portable unit upstairs family room (there is only one vent to the hottest room of the house, and it's a large room). I found it in the classifieds several years ago for about $150. We have loaned it to my parents, when their central air went out, until they could get their unit fixed. It doesn't seem to add much to my electric bill, and is nice to have as a backup (I used it in the main floor with fans to direct air when my AC went out and had to wait to have it charged).
I hate the heat, but those portable units are great.