Yes, temporarily. Many people have. But remembering anything but a minute impression of the experience when back in waking-state, coupled with a language without the capacity to describe it, makes the experience essentially ineffable.Have you personally experienced this?
Why extraordinary? This is the goal, after all, of the "Eastern" religions. It's common enough that Huxley described it as "Perennial Philosophy."That's a pretty extraordinary claim. Because it would imply nothing short of "God-consciousness." (Only God can experience all experiences.)
I would note, though, that "God Consciousness," whilst a real thing, is not the ultimate state of consciousness you're asking about.
God is a thing, and God's usually thought of as different from the other things He created, implying a duality, not the Unity you ask about.
God is just one step in a hierarchy of consciousnesses.Mystics seek to transcend God.