Great question. That question needs to be broken into three sections.
1. Do they keep your drink cold? Yes. See this article.
2. Do they keep your hands warm? Yes, they keep your hands from touching the cold drink.
3. Are they a great party favor? Yes people use them for weddings, birthdays, graduations, reunions, etc. And they are very popular.
4. Are they good for Branding? Yes, Koozies work as a great branding promotional item. they are inexpensive, and stay in the customers hands for a long time.
So, to sum up, Yes, they really work on all fronts. Koozies really work.