When the beta became public, I used the stand-alone installer, which was a 397MB download. Installation went smoothly, albeit slowly. After creating a System Restore point, the entire installation took an hour and required only a single reboot. It proceeded unattended. It was as simple an update as you could ask for.
If you install the service pack via Windows Update, the download is considerably smaller -- about 41 MB.
Some weird anomalies
When you install SP2, you won't notice any changes to Windows Vista. It alerts you that the upgrade is complete. Just to make sure, you can also go to Control Panel --> System and Maintenance --> System, and at the top of the screen, you get a confirmation message. Other than that, though, don't expect to see anything new.
But although I found nothing new in Vista SP2, I did find several odd anomalies. The first had to do with power settings on my laptop. Normally, when I run my laptop, an icon runs in the System Tray, notifying me if I'm using battery power, showing me how much power is left, and letting me change my power plan among Power Saver, Balanced, and High Performance. Right after installation, that power-saving applet seemed to have disappeared. However, after I rebooted, it appeared again.
Equally odd was that when Vista SP2 launched, it showed me that one of the updates available was Windows Search 4.0, even though Windows Search 4.0 is supposed to be part of SP2. Again, though, the reboot fixed the difficulty.
However, the reboot also caused a problem -- Vista no longer recognized my wireless adapter. But when I rebooted yet one more time, Vista recognized the adapter without a problem. The moral of the story: This is beta software, so beware.
The bottom line
Should you install this service pack now? Unless you have some need to do it because you're a systems administrator -- or if you're one of those people who has a burning desire to test new software, no matter its usefulness -- there's no need to try it out now. In fact, you shouldn't try it, because you may run into problems, as I did, with your wireless adapter. You'll be better off to wait until the expected April ship date when it's supposed to be fully baked.
Note: For a list of the most important changes in SP2, see Microsoft's Notable Changes in Windows Server 2008 SP2 Beta and Windows Vista SP2 Beta." You can also find a list of included hotfixes and security updates.