Well, first of all, it's eugenics, a word of Greek origin meaning eu (good) genics (genes). Now the idea of controlling our genes, has been around since ancient Greece, thus it is a very old idea. It is not driven by biology or genetic sciences, it is rather a philosophical / social idea re-appearing from time to time. There are several problems with it. The first is that it deals with the concept of good genes and bad genes. As any bio-scientist knows, that is not something that exists in our field of study. Good or bad is philosophical, not scientific speak. The second and greatest problem, is that if we accept the terms good-gene and bad-gene, than we allow people to define them according to their point of view, which is extremely dangerous. For example, someone could decide that the gene that is responsible for a group's skin color, is "bad", the very definition of racism (actually I have the impression that the Nazis tried stuff like that).
Genetic therapy, which is a closely related thing, is very different because of it's approach on the subject. Genetic therapy tries to bring back to health people that suffer from genetic diseases. That, as is evident, while it might make use of the same technologies, has no moral gray area. So, eugenics, the human attempt to "improve" human genetic code, is a slippery slope, because it cannot be framed in scientific terms exclusively. My take on it is that until science has enough knowledge of the human genome as to be able to differentiate between all shades of gene function and their respective role, eugenics will be a bad proposition.