Converting from one to the other is the problem. Sometimes, I'd get a drawing all dimensioned in metric (aka "Demon-tioned in Metric"), in which case I'd just program my CNC mill in metric, which then required that I convert the cutter diameter to metric, but was generally the easier thing to do. I always multiply the metric dimension by .03937 to get an inch dimension. Some prefer to divide by 25.4, which gives exactly the same answer.
Despite the requirement to buy a lot of new tools, I was in favor of changing to metric back around 1980, the last time the USA was considering making the switch. It's pretty silly that there are only two countries in the world that use the King's system, especially since the King doesn't anymore. There are some stupid things about Metric threads that give me pause. I believe European metric thread forms are a wee bit different than the Asian ones, and that England wanted to use a 55 degree thread form instead of the 60 that everybody else has always used...a holdover from Whitworth, I was told. I think they were convinced not to do that. Silly stuff!

Steve