I installed SP1 about two months ago, when the download sites were originally posted on this forum. No negative issues to report as of yet. I have both the 32 & 64 bit versions saved to a flash drive, have used them on several installs.
The one positive that I can report is that from the time that I press the power button to start my desktop, is that I have internet about 10 seconds faster. Nothing to set the woods on fire, but any improvement is better than none.
Finally, when MS released it's statement about SP1, they clearly stated that there would be no major changes, and that for the most part, it was a collection of what we have, or have been offered. There is an anti-piracy update (KB971033) that many users refused, that's included with SP1.
If there is anything even remotely negative to report, it's the install time of the pack. For those with higher end systems, this time is probably shortened. Mine is not high end (see my specs), therefore it took around an hour.
There are a few things that you can do that may make things better for you. First, make sure that your drive is defragged. Secondly, create a full backup (image) of your computer prior to installing the pack, and create a restore point. Third, disable your AV, that may slow things down, as it scans the downloaded pack, and in some rare cases, may block the install (this I read on MS's install instructions).
After the successful install, use CCleaner to purge your system of temporary system files (uncheck the 24 hour wait option in the Advanced settings menu) be sure to defrag your drive once again (did your son do this?), it will need it, and run a full scan with the AV of your choice (please, don't use AVG, it's problematic with Windows 7). The ESET Online Scanner will be fine to use.
http://www.eset.com/online-scanner
Make sure to run this in IE, as you'll be able to select more options as to how to scan.
If you do these things, although they may be time consuming, you'll have a better experience with SP1.
Best of Luck,
Cat