Last night, I checked my July 2005 power bill with that from July 2004. Both were hot months with a lot of AC usage, and while I traveled a bit this year, I was also on the road last year around the same time. It looks like I used about half as much electricity for July 2005 than for July 2004. My bill was about $100 lower than the same time period, and I suspect most of this is because of my AC system upgrade.
So, only another 44 months of savings like this and the system will have paid for itself :)