We've done I-77 / I-79 about 8 or 9 times now and we prefer that route.
We did I-75 once, but we went through so many cities, and the traffic was NUTS, plus we waited HOURS at the Detroit border crossing.
I DID get a speeding ticket on State Hwy 19 once (Summerville)... $165 for 10 mph over, so beware that. Other than that, no problems.
We normally stop about an hour or so into North Carolina, so we can zip through Charlotte early in the morning.
Really the only cities you pass through are Charlotte and Jacksonville, which is GREAT!
I'm leaving tomorrow for Disney... and I'll be on I-77/79! Honk if you see me