I'd rate the chance that early upload techniques miss some necessary components of sapience as reasonably high, but that's a technical problem rather than a philosophical one. My confidence in uploading in principle, on the other hand, is roughly equivalent to my confidence in reductionism: which is to say pretty damn high, although not quite one or one minus epsilon. Specifically: for all possible upload techniques to generate a discontinuity in a way that, say, sleep doesn't, it seems to me that not only do minds need to involve some kind of irreducible secret sauce, but also that that needs to be bound to substrate in a non-transferable way, which would be rather surprising. Some kind of delicate QM nonsense might fit the bill, but that veers dangerously close to woo.
The most parsimonious explanation seems to be that, yes, it involves a discontinuity in consciousness, but so do all sorts of phenomena that we don't bother to note or even notice. Which is a somewhat disquieting thought, but one I'll have to live with.
Actually, http://lesswrong.com/lw/7ve/paper_draft_coalescing_minds_brain/ seems to discuss a way of upload being non-destructive transition. We know that brain can learn to use implanted neurons under some very special conditions now; so maybe you could first learn to use an artificial mind-holder (without a mind yet) as a minor supplement and then learn to use it more and more until death of your original brain is just a flesh wound. Maybe not - but it does seem to be a technological problem.
A new paper has gone up in the November 2011 JET: "Ray Kurzweil and Uploading: Just Say No!" (videos) by Nick Agar (Wikipedia); abstract:
The argument is a variant of Pascal's wager he calls Searle's wager. As far as I can tell, the paper contains mostly ideas he has already written on in his book; from Michael Hauskeller's review of Agar's Humanity's End: Why We Should Reject Radical Enhancement
John Danaher (User:JohnD) examines the wager, as expressed in the book, further in 2 blog posts:
After laying out what seems to be Agar's argument, Danaher constructs the game-theoretic tree and continues the criticism above:
One point is worth noting: the asymmetry of uploading with cryonics is deliberate. There is nothing in cryonics which renders it different from Searle's wager with 'destructive uploading', because one can always commit suicide and then be cryopreserved (symmetrical with committing suicide and then being destructively scanned / committing suicide by being destructively scanned). The asymmetry exists as a matter of policy: the cryonics organizations refuse to take suicides.
Overall, I agree with the 2 quoted people; there is a small intrinsic philosophical risk to uploading as well as the obvious practical risk that it won't work, and this means uploading does not strictly dominate life-extension or other actions. But this is not a controversial point and has already in practice been embraced by cryonicists in their analogous way (and we can expect any uploading to be either non-destructive or post-mortem), and to the extent that Agar thinks that this is a large or overwhelming disadvantage for uploading ("It is unlikely to be rational to make an electronic copy of yourself and destroy your original biological brain and body."), he is incorrect.