I have used Corey's auto service when I lived in Cumming, GA for 8+ years and I can safely say that I did not meet any more competent car mechanics than this place. It is a family owned business and two brothers in their mid to late twenties run it. They are real polite and nice people with college education, unlike what one might expect from a mechanic. I never had any problems with any service they performed of variety of cars I took to their shop. They are very prompt with their promised times. I never had to go there and wait for my car to be ready at the promised time. Even when I dropped in without an appointment, I was out quite fast.
I strongly recommend this place to anybody living in this area.