Second, Home Sharing now shares pretty much everything in a library: you can copy music, video, audiobooks, and even iPhone apps--you just drag and drop. Of course, if you copy iTunes-purchased content, in order to use that content you must be authorized for it. For example, if your daughter's library is Home-Sharing linked to yours using your account, you'll be able to freely copy an iPhone app she purchased using her account; in order to use that app, however, your Mac will need to be one of the five authorized to use content. This can get confusing when you've got multiple people sharing libraries, each of whom purchased content under a different account.

The obvious appeal of Home Sharing is that it finally provides a mechanism for easily copying iTunes content between Macs, but it also provides a couple features that make it easier to keep multiple iTunes libraries in sync. First, if you select a shared library and then choose Items Not In My Library from the Show pop-up menu, the list of contents will be filtered to show only those items not in your own library. (Confusingly, if you select the main library icon for a shared library, only music is displayed; to display apps, movies, or other content, you must first expand the library in iTunes's source list and then select the desired media type.) You can then copy only the items your spouse has added to his or her library that you haven't yet added to your own.

Even better, iTunes can check for new items in another library and copy them to your own. Configuring this feature is as simple as clicking on the Settings button while viewing a Home-Shared library, choosing the types of media you want copied, and clicking on OK.

This sounds like a great feature, and in some ways it is. But as my colleague Peter Cohen , it also has some sizable limitations. The biggest--and it's a doozy--is that the automatic-transfer feature works only with content downloaded from the iTunes Store. (The text of the Settings dialog, , implies this, but we confirmed the limitation both with Apple and through our own testing.) So while you can copy media ripped from your own CDs and DVDs, music purchased from Amazon MP3 and eMusic, and other content you didn't get from the iTunes Store, that content won't be copied automatically. This auto-transfer limitation would appear to be a nod to music industry concerns about piracy, but for many people, it cripples the ultimate usefulness of a very cool feature. (See Peter's article for a number of other issues.)

It's also worth nothing that Home Sharing isn't a way to maintain a central iTunes library that syncs with multiple Macs or user accounts in the same household. Besides the auto-transfer feature being limited to iTunes-purchased media, tracks copied via Home Sharing don't maintain metadata such as ratings or playcounts, nor can you transfer or sync playlists. iTunes 10, anyone?