I finally replaced my 30-year-old water heater last Monday, with a 6- year version from BOB (big orange box). Did it all myself, for less than $400. Would've cost $600+ to have done.
Anyway, I had an issue before with a signifigant drop-off in water pressure any time I went all-hot anywhere in the house. I still do.
The 3/4" connection pipes on top of the old WH were pretty crudded up, and I thought that was my problem until I tried all-hot with the new heater in place.
The copper pipes immediately before and after the WH are clean as a whistle.
Not so much of a problem now that I don't need to go all-hot to get anything but a stone-cold shower, but still bugs me.