Just ran this on my various blogs:
$ svn switch http://core.svn.wordpress.org/tags/3.0
And navigate to the Admin pages of the blog. Done — WordPress upgraded to the latest and the greatest within seconds. Would be faster if something like git is used instead of subversion (which I found slower and slower than git or hg), but certainly no complain here.
As of WordPress 3.0 — not much changed on the surface. I am pretty sure Matt and gang have done the heart transplant under the bonnet, but these days I am so out of touch with the latest WordPress development I seriously have no idea what has been changed. It does chew up a bit more memory, which can be a problem if you are running tiny virtual servers.
So far so good.