Is there really any important difference between your existence now, and one in which your physical body was replaced by a particle-for-particle copy every so often? To WHOM or WHAT is that difference experienced?

Yes, it is of importance to the me right here, right now, in the present. Under one interpretation I wake up in the other room. In the other I do not - it is some other doppelgänger which shares my memories but whose experiences I do not get to have.

If I somehow find myself in the room with my clone, it's true that there's no way short of checking external evidence like security footage or somesuch to determine which is the real me. That is true. But that is a statement about my knowledge, not the world as it exists. The map is not the territory.

If I were to wake up in the other room with the clone nearby, it no longer matters which one of us is the original or not. He isn't me. He is a separate person that just happens to share all of the same memories and motivations that I have. I want to say that I wouldn't even give this copy of me the time of day, but that would be rhetorical. In some ventures he would be my greatest friend, in others my worst enemy. (Interestingly I could accuratly tell which right now by application of decision theory to the variants of the prisoner's delima.) But even when I choose to interfere in his affairs, it is not for directly self-serving reasons - I help him for the same reason I'd help a really close friend, I hurt him for the same reason I'd hinder a competitor.

The truth has real implications for the me that does exist, in the here and now. Do I spend not-insignificant sums of money on life insurance to cover cryonic preservation for me and my family, thereby foregoing other opportunities? Do I consider assisted suicide and cryonic preservation when I am diagnosed with a terminal or dibilitating disese of the brain? Do I stipulate revival instead of uploading in my cryonics contract, knowing that it might mean never being revived if the technology can not be developed before my brain deteriorates too much? Do I continue to spend time debating this philosophical point with other people on the Internet, in the hope that they too choose revival and there is safety in numbers?