Donor has to be non-maternal relative because maternal relatives share the same DNA composition hence harvesting DNA for donation from them will be of no use. If one could manage a disease-free DNA from maternal-relatives then the genetic composition of the offspring will be 100% from its parents.
Benefit of this procedure is that mitochondrial diseases such as diabetes mellitus or deafness, which are transmitted through maternal mitochondrial DNA, can be prevented in the next generation.
Safety and efficacy of mitochondrial DNA replacement are still undetermined. Social and ethical implications are also alarming in that with this technique germline is altered and this alteration passes on to subsequent generations. Offsprings from this technique might face identity crisis in their future, however it still has to be determined.