Well this is going to be challenging for those of us with current Astra Militarum armies mixing in new miniatures. It could very well be the deciding factor whether or not we pick up these miniatures or figure out how to use the old ones with new rules coming - which I am certain will push the new miniatures.
Painting though will be much easier and proportionally I like the new scale. Replace my army.... No. Add in a mix of scales for my army, No. Laugh at how small old marines look next to new guardsmen.... probably. Move to more 3d Printed guys to use my own scale.... yes.