I have just applied Service Pack 2 for Windows SharePoint Services 3.0 to my test environment (x64 Windows 2003 running in HyperV). The installation took just over 10 minutes and didn't require a reboot. Everything ran smoothly and my Windows Event logs are not reporting any problems.
You can download the service packs for WSS and MOSS from the links on this blog post
Click here to see a spreadsheet of the 34 issues fixed in WSS SP2
Some of my favourites are:
SharePoint does not allow some field names to be changed to their original values.
Users are not warned when their files exceed the maximum path.
List views fail when they are grouped by calculated fields or by lookups to calculated fields.