Talking to Russell, his AMM stock is supposed to be as natural as they can get it, while still selecting for a few traits. AMM has a tendency to swarm quite a bit, and can be somewhat aggressive, from what I've heard. Russell has bred them to be calmer, and swarm less, but there is only so much you can breed for when you are keeping the strain pure.
The kicker for AMM is their resilience. AMM has been introduced in nearly every corner of the planet, and has managed to survive. Granted, they don't always flourish, but they have survived in nearly every environment. You can't say that about the Italians, Russians, or Carni's.
I have also heard that AMM has the POTENTIAL to be more resistant to varroa. That doesn't mean that all of them are, but much like Russians, they show some signs of resistance. Again, or so I've heard.
If I can find the cash, I'd like to get an AMM from Russell for next year to try it out. That and a Moonbeam. I tried his Sunkists for the first time this year and I have nothing but good things to say about them. Truly an amazing strain, I can only imagine what he has done with his other stock.