23andMe: Why is Personal Genetics in DNA Testing Important?

I read an article about DNA testing is going mainstream. What does that mean? What’s the impact?
From Babies Online:
Research studies are pointing to the fact that a person’s genetic make up can determine not only his/her risks for certain diseases, it has also an influence on a person’s response to certain drugs and treatments. Personal DNA information is therefore very important in the so-called personalized medicine
There are some controversial issues that raised by the advancement of DNA testing technology.
I found this article is suitable for DNA testing beginners because of it’s simplified context. Read more about it here.