It doesn't seem like it's such a major feature after all, huh?
FWIW, what I do to keep my computers sync'd manually is that I keep a "transfer folder" of stuff that I know I want to put on the other computer on my desktop. (My computers are sometimes on the same LAN together at home). So if I get a new song, I drag it from iTunes into there, and then delete the copy later when I've copied it. I also keep "recently added" playlists that display the last xx songs added to iTunes on both computers, which helps.
Not perfect, but it works....