"Frankly NX handles mixed units quite well"
Wow, I didn't think anyone would say THAT! In some ways, yes, but its a real pain if you need to relate information in both directions.
Not being able to set a different unit part as the work part is a dramatic limitation, and the workaround of creating another top assembly with different units is inelegant.
More often I have been taking the time at file translation to convert the part to inches.
In some cases, it may work out better to create a second part in inches then wave link the mm body in to maintain associativity. But then you still might have trouble with outputing modifications back to the original source.
NX 5.0.3.2 / 6.0.1.5 MoldWizard