I'm still looking into Moai but am leaning on switching to it.
It has 2 major disadvantages. It definitely is not as polished as Corona -- for example I had to wait several days to download the SDK due to getting various PHP errors on the site until it was fixed. Also, it has a much smaller developer base than Corona, which puts to issue how sustainable it will be. It seems backed by a solid game development company and is getting more mid-sized companies starting to get involved so I think it will be ok but that is a concern I don't have with CoronaLabs.
In terms of its low level nature, so far it seems it actually will be easier for my particular game, not harder. My game is a simulation, and it is overall more geared for that kind of game, such as having a better coordinate system. Another aspect that will make my code much cleaner is Moai supports multiple windows-- something I was very surprised not to find in Corona. Corona doesn't even have recangular sprite clipping built in though some of that infrastructure is on their roadmap and you can do somewhat inefficient hacks using SpireSheets or Masks to handle it for now. So I'm finding, much as Serapth suggested, that the low-levelness of Moai is actually in many cases a big advantage instead of disadvantage, for experienced programmers at least.