While I certainly wouldn't use that material for new construction, the old stuff has held up quite well in our part of Southern California. The pipes in our house were installed in the late 1950's and the couple of pieces I have been able to see (one pipe cut to replace a valve, the ends of the pipes to the sprinkler valves) are rusted, but only a thin layer. No sign of the "packed with rust" that everybody talks about. I'm guessing that the lack of aeration (usually) combined with the hard water is somehow protective. That said, the main line we replaced, which was of course buried in dirt, was pretty nasty looking on the outside. It was not, however, leaking. It was replaced because the valve at one end was stuck open and the one at the other end, next to the city's shut off was dripping out of its handle. We put thick copper in to replace the galvanized there. When we do the house that will be PEX.
All of the galvanized pipe should be replaced.
