We are also at Decatur Montessori and my son is thriving. There are things about the school that drive me crazy, but I definitely love their committment to Montessori and it really is a fantastic environment for DS.
I also have a friend whose DD attends Arbor Montessori and they LOVE it. LOVE. We actually checked out their program too and were very impressed. Problem is Arbor doesn't go year round so that didn't work for us.
I also have several friends with children at Montessori at Emory(not actually at Emory, is located on N. Decatur Rd) and they are very pleased with their choice (actually defected from Decatur Montessori and switched to Emory). We also checked out that school and while we loved the director and thought there were many positives, it didn't feel "Montessori" enough for us (although it certainly incorporated many Montessori elements and if you aren't completely Montessori-anal it would definitely appeal to most parents).