Coffee Joulies work with your coffee to achieve two goals. First, they absorb extra thermal energy in your coffee when it's served too hot, cooling it down to a drinkable temperature three times faster than normal. Next, they release that stored energy back into your coffee keeping it in the right temperature range twice as long.Well, Marco Arment the creator of the fabulous Instapaper bought some Coffee Joulies and ran them through some tests. His conclusion: they aren't worth two dead flies.
I could do more tests with different conditions, but I honestly don’t want to spend another four hours to reinforce what seems pretty clear already: Coffee Joulies do work, but their effect isn’t very strong, and it’s nowhere near their claims that the drink “will be ready to drink three times sooner and will remain hot twice as long.” In fact, the effect is barely noticeable.
I imagine they’d fare well in an all-day test in a vacuum mug or bottle, but the effect is going to be similar to my insulated-mug test: an improvement, but not by much.
UPDATE: The makers of the Coffee Joulies have responded in the comments. They say, "We have run our own tests in literally dozens of scenarios, and the performance varies from greatly exceeding our claims to situations like Marcos"
Coffee Joulies review (Thanks, Jeff!)