I spent several days to test 123D, and in my opinion, it can become a very good companion of MOI.

The good of 123D :
In fact, 123D uses the same engine as Inventor/Fusion, a robust CAD application with an history tree. This means that when you create a feature like a bevel, you can later edit this feature, and the object is remade to reflect the modifications. Moi, on the other hand, doesn't have this tree.
Another very interesting feature of 123D is the ability to push or pull faces of a finished model, a thing that you can't do easily in MOI, even with the "point edition". In the same way, you can at any time rotate faces in 123D.

The bad of 123D :
As a lot of CAD engines, to create a curve, you have first to create a plane, then create your 2D curve on this plane. And 123D doesn't allow you to modifiy these curves in 3D as MOI.
The process to create features as a loft in 123D become very difficult : you have to create numerous planes, then your curves, and so on.
In MOI, on the contrary, the process is efficient, simple, clear, and much more powerful than the one in 123D.

So ? You can obtain best of both worlds : first I create my object in MOI ( without features like bevels that can produce bad models at export ), and then, I can edit them further in 123D, by moving faces (even curved ones) and rotate them as well. In two words, 123D adds a bit of "interactivity" in the process of making an object.

I'm not a fan of Autodesk. In fact, I think it's one of the worst 3D company in the world. But with 123D, they released a powerful and free tool. Even bad companies can do good things sometimes. :)
On the other hand, to say the truth, I'm a big, big fan of MOI, I think this software is a kind of "miracle" in the jungle of heavy, expensive NURBS softwares.

It's clear that 123D isn't able at this time to replace MOI; it can't beat the power and efficiency of MOI, and it is very far from it. But 123D can add some useful functions MOI doesn't have... at this time.
Worth the try in my opinion. :)

Alexis - www.3dsaloon.fr