Infoworld has posted two articles: one is a review of Mac OS X 10.5, otherwise known as Leopard, and the other is a recommendation that businesses avoid Windows Vista.
My experience with Leopard, while not as thorough as the review, has been quite positive. Leopard has been stable and has some must-have features (Spaces, mmmm).
My experience with Vista… mirrors what the Infoworld article says.