Soft water brings out all the flavors in a tea, both good and bad. So if the tea and the brewing technique are top-notch, this will provide best results, otherwise your results will vary.
Hard water releases flavors selectively from tea. A tea that might taste unpleasant using soft water will probably taste alright using hard water, because many of the unpleasant flavors will remain in the leaves. A delicate tea might taste bland in hard water for the same reason, because many of the pleasant flavors remain in the leaves. Lu Yu, the ancient Chinese tea master, believed that hard water (water from stalactite springs was his term) is the best choice for tea. On the other hand, in his day the treatment of tea was much harsher, and it was trickier to control the brewing variables.