It is a very complete format with lots of different options for different programs to interchange model information.
Is it any better then x file, right now in the betas there are a lot of issues coming from different programs giving it a hit or miss results when trying to import different flavors of FBX into the editor.
It will need to be ironed out for sure, as very little people use Maya in this engine.
The success of multi textures will require a shader that will be able to render each texture and then be able to pass that info to the renderer. So no quarantine this will make multi textured models easier to deal with or render proper.
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