Just had a stack of 4 PCBs back and one PCB has the mounting holes out of alignment by almost 0.5mm so is scrap. I realigned all key components between the PCBs about a week before shipping but I (and I assume many others) have to switch from mm to mils and back constantly because of a mix of SMT, thru-hole and both metric and imperial mechanical components (e.g. 2mm and 2.54 mm connectors !), and the rounding errors or whatever causes the drift is a disaster.
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
