Copper in our house is 55 years old. I had one spot spring a pinhole leak last year but otherwise it has been great. Granted, our city water is neutral and old copper was thicker than the new stuff.
Remodels and rework in my house are part of a gradual conversion to PEX. It's good stuff.
With copper, you can size runs according to the ID of the pipe, and the fittings are largely negligable. With PEX, you SHOULD use as few fittings as possible and size according to the ID of the fittings on any given run. Note, high velocity (throttling) through fittings makes a run noisy.
There are good PEX jobs, and there are cheapo jobs, where they undersize everything and use plastic fittings. Use brass and size accordingly and it's a good system. Even better if you PEX-A and do expansion fittings. Make sure you consider growth/contraction of the lines and take appropriate measures to keep them from being noisy as they cycle.
One nice thing about PEX is that it is flexible. You can completely avoid in-wall fittings with some planning ahead. (Shower/tub valves are really the only exception.) I hate walking through a basement and seeing completely avoidable pex 90s everywhere, knowing there's a bazillion questionable and flow restricting crimps behind the drywall.
My biggest issue with PEX is that it allows low skill plumbers to create a really junky but leak-free product with a minimum of overhead (tools) to get into the trade. Fast and cheap. But I guess that's the case with PVC/CPVC too. Copper sweating at least required some skill and planning.
