Windows Vista Service Pack 1 went live yesterday. It's available for those who want it via Windows Update. It won't automatically download to your machine until April, but users can get it now. I did so and, after you finish cleaning up the coffee you spit out for me being an early adopter, you can read further of my experience.
First of all, the update, like the operating system, is large. It's a minimum of 120MB. That will take some systems a little while to download. My machine was up to date on all of it's patches and such so my download came in at 121MB, but the website warns that updates might reach 400+MB (I assume that this is a rare case where no patches have been applied yet).
The update went smoothly. One of the new functions of Vista is that some updates don't complete until a restart has been commenced. This actually was true of older systems as well, but they didn't have a splash screen informing the user as to why the shut down was taking so long. Vista does that and it's good, but being new it might confuse some users as to why, after Windows Update claimed that the update was complete, are they now getting a screen on shutdown saying that the update is finalizing.
Lastly, when logging in, each user is faced with a warning message that reads something to the effect of "Windows Mail has encountered an error and is shutting down". A similar message appears for IE8. What both programs are really doing is finishing their updates and adding in the personal settings. However, the dialog box doesn't explain that. It's needlessly worrying to the user and may cause some panic. How do I know this? Because I was drug out of bed last night by the co-signer due to the messages. To be fair to her, she's been experiencing other odd problems with the OS before the update (her profile disappeared and appeared to lock) and these dialog boxes could have been related for all she knew. Like I said, needlessly worrying. Why not say, "Windows Live Mail is finishing updates" or some such?
Otherwise, clean install. It's working fine thus far.