There have been a few discussions the past year or so about AMA. The general sentiment doesn't seem very positive, but there aren't as many discussions about them as there are for better known companies like PSA, JSA, GAI, PAAS etc. The fact that they haven't really established themselves one way or another after 10+ years may be a good indication that they're worth avoiding.
They're not a trustworthy autograph authenticator in my opinion. I've seen a clearly secretarial Elvis signature sell from them and even called them myself and was told they used forensic evidence to conclude 100% that an autograph is authentic; that's all rubbish.