I installed the version initially posted 29 Apr'08:
http://arstechnica.com/news.ars/post/20080429-microsoft-releases-the-long-anticipated-windows-xp-sp3.html
I installed it in two computers, one running XP MCE, another running XP Pro. SP3 installed in 10 minutes. I have encountered no significant problems, but others have reported [i]crashes[/i].
That said, Microsoft has published a number of bulletins regarding troublesome SP3 installations:
http://support.microsoft.com/ph/1173
So I recommend that you make a complete backup of your OS configuration and data files. SP3 will actually do this automatically for you, if you don't have imaging software such as Norton Ghost.
Once you have installed SP3 and you have determined that it's working OK, you can delete the SP3-created backup file. [i]Beta[/i] tests have shown a 10% enhancement in performance with SP3 as compared to SP2, and SP3 includes certain Vista components for additional reliability.
In summary, go ahead and install SP3.
Rick/Portland, OR