You'll honestly have to tell me. As I was unable to attend AM^2, I have no idea if I would have called it a successful venture. A friend of mine who attended said they succeeded in going 24 hours all 3 days but the con felt "small," especially in comparison to AX.
Chase Wang seems to feel that his venture was a huge success, touting 8,000 attendees showed up for the weekend. Ref. It certainly sounds as though AM^2 will push for another showing next year, although this has yet to be confirmed (as opposed to AX which is already offering badges for 2012).
Again, I can't express how much Wang will choose a different weekend. Even if AM^2 remains free it will still cost gas/mileage and extra parking fees to attempt to attend both cons. Anaheim is close to Los Angeles, but not that close. Also, I would want to fully enjoy both cons and not feel like I'm some child picking a side of who I like best. It feels...unfair.
Original thoughts on AX vs. AM^2 here (includes background info on AM^2).