What is masked by some of the hoopla around cancer is exactly this, gene therapy is advancing, and it can rewrite your DNA in-situ. This meets pretty much every definition of the term 'game changing' from atheletes who want the Sherpa gene for making more red blood cells to a host of genetic disorders that lead to a myriad of complications in the patients life. Some resarch has been marred by some very serious negative outcomes [1] but if it can be mastered, the world will not be the same, for any of us.
It's important to note that they didn't release HIV into the patient in vivo. They infected T-Cells in vitro that then multiplied in the patient's body. By doing it in a test tube they can also throw out the bad results. Gene therapy isn't going to work quite yet inside of a body, but if you can grow replacement organs from scratch, you can definitely get somewhere.
Here's what I don't get about your hypothesis... very rarely is there a single gene for specific trait in humans, right? Usually it's a combination of genes, and often the trait produced is the effect of their impact on embryology.
So wouldn't it be impossible (and possibly extremely dangerous) to attempt to change many traits by performing in-situ DNA rewrites?
As I understand the current research, its focussed on 'fixing' problems where one link of the chain is broken. So when a specific set of enzymes are missing (think lactose intolerance) or immunodeficiencies.
Wholesale rewriting chunks of DNA would, as far as I can tell, likely kill the patient. That being said, the potential gain for some to have larger changes to their DNA will no doubt compel someone to try it. A biologist that gave a tech talk at Google on the technology said he had received calls from althetic trainers asking about how his work might be applied to other areas. He was convinced that there was already some efforts at using gene therapy to 'improve' some athletes capabilities.
[1] http://www.medpagetoday.com/Genetics/GeneralGenetics/6275