I learned about PEX in 2005, when I met a plumber who built celebrity and zillionare houses in places like Vail. He said all these mansions were all built exclusively with PEX. At the time, PEX was pricier than it is today.
I replaced my copper with PEX about three years ago. The reason I replaced the copper is I was getting pin hole leaks in the copper.
I use PEX A, which requires a Milwaukke tool. A ten year old with a few minutes of training can make a less leak prone connection with PEX A than a Chicago union plumber with 30 years experience with copper. Yes, PEX A is that good, and easy, and affordable.