Cloning is a controversial matter due to inherent ethical, philosophical and religious questions involved, such as diminishing the uniqueness of the individual and the chance of biological failure, as seen in mammalian experiments. A cloned human being's family, for example, is unlikely ever to see him as the original.
The most concrete argument against cloning is that it is still a nascent technology that is prone to high rates of failure. It is plausible that cloning and related genetic engineering could lead to objectification of human beings by the potential to change undesired traits or start completely anew. In addition, the "slippery slope" of legalizing and accepting cloning as a society may lead to other radical technologies that compromise identity.