Brink Lindsey has to be the best writer on the cloning controversy I've read anywhere. His prose is clear and cogent, and he is honest and civil to boot. In one paragraph, he goes to the heart of the issue and sweeps away the legal arguments (like the Slippery Slope):

But if we allow the destruction of embryos created for no purpose whatsoever (as in the case of abortion and, more exactly analogously, the use of IUDs), why should we not allow the destruction of embryos created for a very good purpose -- namely, the alleviation of suffering? Until we are at the point of having serious questions about the personhood of the embryos involved (and I believe such questions arise relatively early in a pregnancy), it seems to me that the case for legalized therapeutic cloning is stronger than the case for legalized abortion. Indeed, it is stronger than the case for the presently humdrum, unquestioned legality of IUDs.