My 2 cents..
I had a house fire in 2006, requiring a complete gut and rebuild, including electric, plumbing, walls, flooring, joists, ceilings, etc. I got several quotes on the plumbing, and all quotes were for pex. So when I decided to do it myself, I used pex. I have done a fair amount of copper pipe work. There is no comparison between the two methods. Copper is labor intense, and very expensive for the copper itself. For a 10' length of 1/2" copper, it was $16 retail. That is $160/100'. I paid $25/100' pex retail. The fittings were about $3-5 each including the clamps. The crimp tool and cutter was $100. The labor savings are huge. Running the pex was easy. I had 0 leaks on pex fittings. Try that with copper! For me, pex is the only way to plumb supply lines. Of course, there are some spots that must be copper, such as hot water heaters and boilers and registers.