I live in Atlanta and we have family in Florida so I am well familiar with the drive. Back when we were DINK's (double income, no kids), I insisted that we fly anywhere where the driving time was over six hours. Now that we're living on one income and we have an additional fare to purchase, we don't fly unless the driving time is over 12 hours.
That being said... we have definitely experienced some of the traffic delays described above when traveling during peak holiday weekends. But most of the time, it's fine. The bottleneck is usually the southern part of Georgia where there are two stretches on I-75, totalling 80 miles, where there are only two lanes in each direction. For the rest of I-75 there are at least three lanes in each direction.
Microsoft Streets & Trips estimates the driving time as 8 hours, 24 minutes from our house in Marietta. That's via my preferred route of I-75 to I-10, I-10 to I-95 in Jax, and I-95 to Exit 205. Depending on what time you're leaving, you may want to consider stopping for the night either in St. Augustine or Daytona Beach.