Forgot to mention this earlier, but I’m happy to say that I installed Windows Vista Service Pack 1 last night:
The installation was pretty smooth, it seemed to take about 15 minutes to decompress everything (which it did from within Windows) and then it installed in about 40 minutes after a reboot. Everything came up as it was supposed to, and none of my custom firewall extensions got disabled (as other Windows Updates have done). One weird thing is that I never saw this update as an optional Windows Update from within the Windows Vista interface, but actually had to download the standalone installer from the Download Center. I’m told this can happen for a variety of reasons, but I’m not sure what they are.
As for performance, I haven’t really noticed any difference (although I haven’t been benchmarking file-copying or network shares). I’ll keep you posted as I experience more with SP1, as its supposed to get pushed to all Vista users as a recommended update in April.