I live in Birmingham, AL and go to most of the C's games in ATL dating back to the mid 80's. There are almost as many C's fans at these games as there are Hawks fans. I've been to several in Boston and there is no comparison to the crowds. The Garden has by far the most knowledgeable fans in the world. I've also been to New Orleans which is thought of as rowdy and it doesn't compare either.
Atlanta is a nice city, but it's sports fans are flighty. This is why I found Bibby's comments last year to be so moronic. The Hawks team, like their fans get up for "big games" whereas Celtic fans understand that it's just another test along a journey to something more important...even when we are not very good.
Phillips Arena is all rappers, fire, stripper-like dancers and PA announcers that start chants of "Defense!" during the ****' game. Enjoy the trip to Atlanta if you go, but expect a circus as much as a game.