Let me say, PEX is a joy.
I had never used it in any capacity and was pretty nervous about tackling the whole house in one shot without the relative comfort of copper and sweat joints. Add in a toddler who values her nightly bath and I had my work cut out for me. Or so I thought. Did I mention PEX is great?

With future renovations planned for bathrooms and the kitchen, I didn't want to be opening walls, which limited me to the only area of exposed piping, in the ceiling of the laundry room. I went with a Viega manabloc manifold and crimp PEX fittings. Friday night I shot all galvanized threads I was going to try and tie into with some WD40. Saturday morning, after dragging around, I cut water off at the street. About 1 hour later, I had sweat the 1" copper into the house into PEX and a new ball valve. Water back on and no leaks. Encouraging.

Another hour had the whole house filter in and tied into the manifold.

The 1" PEX is not very flexible and the fittings I had were a tight fit. My hands were looking forward to the thinner pipe.
The water heater took a bit (of a hack) to get to as the tankless isn't in yet. Old house has the tank heater with the chimney on the opposite side of the house. Not fun, but hooked up to copper 3/4" with some Sharkbites as it'll just be a temporary run. Still, better than the almost choked closed 3/4" galvanized into 1/2" PEX into 3/4" copper that had been in place. Crazy.

With those systems in, I started on the galvanized with wrenches and reciprocating saw. Slow but steady progress...and wow were the old pipes gross. Glad for every cup of Brita filtered water I'd enjoyed, but thinking only of those which I'd had direct out of a faucet. Yuck.

I ended up tying into 1/2" PEX into 3/4" hot/cold trunks for two bathrooms on the second floor, one copper and the other galvanized. The galvanized is next on the list for renovation, so just needed to unblock those baths until we open walls. Not perfect, or even legit, but as one will be redone correctly in soon and the other is all low-flow fixtures, I patched in and moved on.
Kitchen sink and dishwasher along with half-bath on first floor are all homerun with PEX. Washer and laundry sink as well. Quick and easy.

I still need to put supports on either side of the manifold to support each line and UV film on the windows. Then a basement wall to frame in, add laundry box, and the tankless heater on that new wall. Good times.

Very happy I didn't go with copper for now. Hope I feel that way in 10 years.

The need to use all those 90 degree fittings makes this NOT a good spot for PEX.

Use the PEX when its flexibility is a benefit.

Keep in mind also you do not have to remove the old galvanized lines.
Just cut them off as is convenient.

You might blow them out just to avoid any trapped water draining out at an undesired time and location.

Galvanized lines that'll be orphaned with bath remodel run up outside wall - intended to leave them in. Good idea to shoot some air through to help dry them.

There sharpness is also going to cause some pressure drop when the water is flowing.

They look a lot sharper than a wrought copper bend (especially a long bend).

That said, these are the only elbows in the system, and certainly fine on pressure. In studying up on PEX, found sources like the one linked that talks about "PEX tubing has a smaller inside diameter than copper tubing, at a given source pressure, both tubing systems meet the farthest fixture demand." That study had a couple PEX 90s combined with bends.

Definitely the 1" PEX 90s look pretty wimpy next to a copper 90.

Just plain sloppy.

If they used that many 90s in the PEX setup they are crazy.

One of the advantages of PEX is that when space is available for bends they can be made without any fittings.

A general test like they have done just shows that the two technologies when configured similarly can flow about the same water.

It says nothing about any particular system, and without flow curves you cannot determine if a particular system is going to be adequate.

While few DIYers seem to ever use anything but type M copper (hard drawn and not bendable) type L can be a great tool (especially in old work) since it CAN be bent and formed.

It takes some practice (even with the correct bending tools) to not collapse the pipe, but it can save fittings in many cases.

Copper:

PEX:

So the 1" PEX 90s add a bit more than double the length of pipe vs. copper 90s.
PEX manufacturers make various claims to lower friction than copper, while the ID of PEX is less than equivalent OD with copper. So if I net those out, I can take my 4, 1" 90s, each roughly equal to 10' of 1" tubing. At 8 GPM, 1" PEX has a PSI/FT Pressure Drop of 0.041, giving me a 1.64 PSI drop.

That sound right? Sounds like at some point I should take the 10 minutes to change these 90s out for bends.

I did have one problem and from what I was told, PEX likes to go back to it's original shape. The plumber put a loop under the water supply to my clawfoot tub, and I came home one day from work to find my oak floors in the dining room flooded. Luckily most of the water was flowing down to the unfinished basement below, and the hardwoods were fairly easy to clean up and dry. I discovered the PEX had tried to go back to it's shape, pulling the supply line just off enough to cause the leak. I fixed it by adding blocks near the PEX and securing with u-clamps to keep the shape the plumber intended, which in turn stopped the leak. I'm not sure if this is a common problem or not, but even with that problem, I love my pipes!

Also regarding the galvanized. I've found that it cuts really well with a cut-off blade in an angle grinder.
Good luck.