DaveP:
I think we can't really say that OS4 is better in usability than OS3.9. It's too early to say:
because it'll depend on the stability of OS4 (many crucial new parts on new hardware); on the compatibility (emulation)...
This is really a very different product.
Right now MOS lacks a few details, but has new feature and is very compatible and stable.
But of course time will tell and we must all remain open and cheerful
