It is necessary to separate reasonable extrapolations and speculation about mind uploading from the magical thinking surrounding it. Several metaphysical questions are brought up by the prospect of mind uploading. Like many such questions, these may not be objectively answerable, and philosophers will no doubt continue to debate them long after uploading has become commonplace.
The first major question about the plausibility of mind uploading is more or less falsifiable: whether consciousness is artificially replicable in its entirety. In other words, assuming that consciousness is not magic, and that the brain is the seat of consciousness, does it depend on any special functions or quantum mechanical effects that cannot ever be replicated on another substrate. This question, of course, remains unanswered, though considering the current state of cognitive science, it is not unreasonable to think that consciousness will be found to be replicable in the future.
Assuming that consciousness is proven to be artificially replicable, the second question is whether the "strong AI hypothesis" is justified or not: If a machine accurately replicates consciousness, such that it passes a Turing Test or is otherwise indistinguishable from a natural human being, is the machine really conscious, or is it a soulless mechanism that merely imitates consciousness?
Third, assuming that a machine can actually be conscious (which is no great stretch of the imagination, considering that the human brain is essentially a biological machine), is a copy of your consciousness really you? Is it even possible to copy consciousness? Is mind uploading really a ticket to immortality, in that "you" or your identity can be "uploaded"?
Advocates of mind uploading take the functionalist/reductionist approach of defining human existence as the identity, which is based on memories and personalities rather than physical substrates or subjectivity. They believe that the identity is essential; the copy of the mind holds just as much claim to being that person as the original, even if both were to exist simultaneously. When the physical body of a copied person dies, nothing that defines the person as an individual has been lost. In this context, all that matters is that the memories and personality of the individual are preserved. As the recently-murdered protagonist states in Down and Out in the Magic Kingdom, "I feel like me and no one else is making that claim. Who cares if I've been restored from a backup?"
Skeptics (at least those who criticize on rational, materialist, non-dualist, non-religious grounds) question if it's possible to transfer a consciousness from one substrate to another, and hold that this is critical to the life-extension application of mind uploading. The transfer of identity is similar to the process of transferring data from one computer hard drive to another. The new person would be a copy of the original; a new consciousness with the same identity. With this approach, mind uploading would simply create a "mind-clone" - an artificial person with an identity gleaned from another. The philosophical problem with uploading "yourself" to a computer is very similar to the "swamp man" thought experiment in which a clone is made of a man while the "original" is killed, or the very similar teleportation thought experiment. This is one reason that has led critics to say it's not at all clear that the concept mind uploading is even meaningful. For the skeptic, the thought of permanently losing subjective consciousness (death), while another consciousness that shares their identity lives on yields no comfort.
Consciousness is currently (poorly) understood to be an epiphenomenon of brain activity - specifically of the cerebral cortex. Identity and consciousness are distinct from one another - though presumably the former could not exist without the latter. Unlike an identity, which is a composition of information stored within a brain - it is reasonable to assume that a particular subjective consciousness is an intrinsic property of a particular physical brain. Thus, even a perfect physical copy of that brain would not share the subjective consciousness of that brain. This holds true of all 'brains' (consciousness-producing machines), biological or otherwise. When/if non-biological brains are ever developed/discovered it would be reasonable to assume that each would have it's own intrinsic, non-transferable subjective consciousness, independent of its identity. It is likely that mind uploading would preserve an identity, if not the subjective consciousness that begot it. If identity rather than subjective consciousness is taken to be the essential, mind uploading succeeds.
 See also
- Collection of articles on mind uploading
- Welcome to Life - The Singularity, Ruined by Lawyers.
- Against Naive Uploadism: Memory, Consciousness, and Synaptic Homeostasis, Seth Weisberg, H+ Magazine
- Why Transhumanism Won't Work, The New Atlantis (A misleading title, the article is about mind uploading, not transhumanism in general)