I just finished installing Windows 7 SP1, and in case you haven't taken the plunge yet, I'll tell you this—just believe the hype. Everything Paul Thurrott wrote about the SP1 beta holds up. It was about 60MB for me, but that was including a Silverlight update—I think the last Windows SP I installed made me do it separately from any other updates, but I can't really remember.
Anyway, even on the fairly underpowered notebook I use, the background download and installation didn't slow anything down. The machine had to reboot twice, and it was about 10 minutes total from when I clicked restart to when the machine was logged in and usable again. Everything is exactly as it was, as far as I can tell, other than what the System Control Panel applet reports. There really isn't anything else to say.
Q. Can I upgrade from the Windows 7 SP1 beta to the Windows 7 SP1 release candidate?
Windows 7 Service Pack 1 Beta
Microsoft Releases SP1 for Windows 7, Server 2008 R2 to Manufacturing