No problems here... Upgraded two systems the day it came out, no problems at all. People who buy HP systems with AMD CPUs might have a different story to tell, but IMO, if you buy an HP system, you're taking a gamble that the thing will work properly anyway.
I would suspect that the majority of people who have problems are IE users who have a malware problem they were unaware of. A substantial update like a service pack can, and often will, cause serious problems with malware. Which, on a vaguely related note, is part of why I tell people not to use IE.
If your system is malware free, you have very good odds of it installing without a hitch.
Here is my one simple question. How trouble free is SP3 on my computer?? Some of my friends regret the day they upgraded from SP2 TO SP3. On my automatic update i usually uncheck SP3. I guess is due to what i was told. So i am now asking the experts