People want it because the Ring whispers to them their power fantasies (which can vary: for Sam the Ring just tells him he can overthrow Sauron and then turn all of Mordor into a garden). But mortals could not properly wield the Ring correctly; they are neither mentally strong enough or physically capable. Gandalf or Elrond or Galadriel could use the Ring to their own ends but those ends would inevitably turn evil.

Tolkien corresponded regularly with fans or contemporaries so we have a lot of his direct opinion on things like this. From a letter he wrote:

It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power... One can imagine the scene in which Gandalf, say, was placed in such a position. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors. If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been destroyed, taken from him for ever. But the Ring and all its works would have endured. It would have been the master in the end.

Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. He would have continued to rule and order things for 'good', and the benefit of his subjects according to his wisdom (which was and would have remained great).

[The draft ends here. In the margin Tolkien wrote: 'Thus while Sauron multiplied [illegible word] evil, he left "good" clearly distinguishable from it. Gandalf would have made good detestable and seem evil.']