It has been officially available for a couple weeks now, but some of us have been running it for over a month at this point. Of course, outside of the MSDN and TechNet avenues, the only way to get it was due to a Microsoft mistake.
In any event, my home and work systems have seen tremendous benefit from the upgrade to Service Pack 1 for Vista, even though there are many who still don't like it. Not only did it improve performance -- especially on my laptop with 1GB of RAM (which ran more slowly on Vista x64 RTM than it did under XP x64 SP2) -- but it also took care of an annoying bug with the task scheduler where it would not properly process the quotes, which broke the application. I checked the list of updates included in Vista SP1 and could not find and entry for this bug, but I verified that the problem existed in multiple Vista RTM systems, both 32-bit and 64-bit. The handle leak in LSM.exe problem was thankfully rolled into SP1, as that one bugged me quite regularly.
You can also read about some changes that were made to the File Copy algorithm due to complaints about the original version of Vista.
I did not experience any of the well-reported SP1 installation woes during any of my five upgrades, a mix of 32-bit and 64-bit systems. From my perspective, this service pack addressed more than enough of my needs to be useful. I'm also sure that it won't do much to change the general anti-Vista sentiment that exists, but it works for my, so I have no complaints...
(The only annoyance was uninstalling all those language packs I had installed but didn't actually need.)