Unlike the og Galaxy S the SiRF chipset is improved as even the Nexus S shared from the Galaxy S its GPS woes. In moderate cover I get within 3~10 meters of where I am. From cold boot to locating myself it takes less than a minute in an Urban Jungle like Downtown San Francisco or Los Angeles.
Areas with sparse GSM/ AT&T coverage may take a little longer for the map to update as the phone usually provisions aGPS before the GPS signal. Its an alternative but GPSfix only takes up 400kb and only helps speed the acquisition process.
Having done a little research but more reading and the iPhone uses the Broadcom chipset whereas Samsung, RIMM, Motorola and Sony been using SiRF. Not that it matters but its a whole mess or probable causes that mess with triangulation; location, density, carrier/wifi signal, and radio interference. Sun spots cause natural EM disturbance and wreaks havoc on all electronics, this could be the cause for some.
I digress...