It can transition easily and legally to existing copper through adapters or even push press fittings such as shark bites. It's very resistant to freezing and corrosion. If you're doing very much of it, you'll want to choose a brand and invest in some crimpers for that brand. I like Viega best but the crimpers will cost up to $200 each and you'll likely need 3/4" and 1/2". The plumbing code we use in Austin says the PEX pipe should be supported every 30" which if nothing else is a good rule of thumb to keep your pipes from rattling. It's also a good idea to insulate as much of your hot water pipe as you can for the sake of energy efficiency. Chances are if you're keeping any old copper pipe, your work may dislodge some of the calcium buildup inside the pipes. That buildup could reduce water flow. You'll want to make sure and flush some of that out by running water through an outside hose bib when you turn your water back on. Also, it'll help to clean your aerators and shower heads afterwards.