As promised, here’s an update. Mavericks is behaving well, very well actually, on an iMac (with 6 GB RAM) and a Macbook Pro (4 GB RAM). If you have MacKeeper, get rid of it first.
The installations went smoothly, although they take time, and the OS is a free 5+ GB download. After installing Mavericks, I added free Sophos anti-virus protection and it plays well with the new OS.
Anti-virus measures were not necessary on Macs when they represented only a tiny portion of the computer population out there. Nobody seemed to bother making malware for Macs. But Apple stuff is everywhere these days, and besides, you could forward email cooties harmful to Windows friends, even if they don’t hurt you.