Knuckle Up has nice facilities, but they really push you on the contracts and the hard sell. It's pretty aggravating, and they have a reputation for being a little on the assholish side to newbies (this was the Kennesaw location back a few years ago).
The problem with Knuckle Up is they only have one world class instructor spread across all their schools, another black belt (Steve), and then I think the rest are brown belts.
Alliance BJJ has better overall cred, but Jacare comes off as very old school -- if you train elsewhere or attend another seminar or even go to someone else's competition, he can get pissy. He's also rude at competitions that I've seen.
I have heard nothing but awesome things about Roberto Traven at Unit2 in midtown. Finally, there's Creighton (a Renzo black belt) who is good, and Eddie Camden at Tiger (he's a Pedro Sauer black belt).
Out in the NW Atlanta area (Cobb/Paulding) there's Alpha Fighting Systems in Hiram, where the head instructor is a brown belt, but that team has been fucking CRUSHING everyone else in the area. It's crazy, they've been accused of sandbagging and haven't even been open for a year (!). At the Bud Cup some of the guys got pissed and competed in blue belt and ended up winning as well -- as white belts. They also took back the team trophy from Bud Cup.
So if you don't care so much about 'name', Alpha Fighting Systems is a good place to train.
Finally there's Fusion Fitness and MMA, which is strictly no gi and very small -- it's pretty much private lessons or so it feels, with about 3-6 people in a class. Limited schedule though (M/W 11:30, T/R 6:30, S 11:30).