I bought one after seeing it at the NHC, but anything similar would work, too:
I have 2 of these now and in my 78 degree house in the summer, 2 quart frozen bottles keep my temps 65-66 by changing them out every 12 hours or so. Last fall, while still warm in my area, I was able to keep temps right at 60 with 4 bottles going after a few days to reach that temperature.
This week, I put one bag inside the other to see what would happen and my fermenter did fit and 1 quart bottle keeps it at 68 changing every 12 hours.
For me the trick is to chill my beer (with a pump and ice after using tap water in the chiller) to 65 to get started. Then when fermentation gets going and wants to go above 70, I've already had 2 frozen jugs in place to keep things below 70.
I've used them almost 2 years now.