I've installed Vista SP 1 once before without a problem. SP1 itself was uneventful, and I should have taken that as a sign to stay away.
Because the second time I tried installing it--on different machine running Vista Home Premium x64--it presented me with repeated blue screens of death.
Finally got it to boot by running "Safe Mode with Command Prompt" (the other safe modes did not work). Now it's been stuck in the "Installing service pack, stage 3 of 3", spending about 45 minutes to get from 42% to 53%. Let's hope System Restore can get me out of this mess. If not, I'm going back to Vista 32-bit, RTM edition.