[The Saucepan Man]

Of course the Nine Rings bestowed increased power on the Men who took them. That is rather the point of the Rings. They wouldn't have taken them otherwise. At first, they were able to use this power to further their own desires and so gain political power, wealth etc. Ultimately, however, the Nine Rings bound them to Sauron and, as davem has pointed out, their enhanced power was directed solely towards furthering his will.

Whether or not they became more powerful than Elves is not really the point. The fact is that they became Nazgul and that surely is not a desirable state of being, however powerful they became. Given their time again, with the benefit of hindsight, do you think that they would have made the same choices?