Apple has released iTunes 10.5.1, which finally unveils the overdue (see “ ,” 6 June 2011). The iTunes Match service, which costs $24.99 per year and is currently available only to U.S. customers, enables you to store your entire music library in the cloud and then play it from any of your computers or iOS devices.
What sets iTunes Match apart from services like (see “ ,” 2 April 2011) and the limited-access  is that iTunes Match doesn’t require you to upload all your music. Instead, iTunes Match scans your iTunes library and uploads only those of your songs that it cannot match with songs in the iTunes Store. For tracks that do match, iTunes Match simply connects them with Apple’s copies instead of uploading, saving you vast amounts of time and bandwidth during setup, and saving Apple vast amounts of storage space that would otherwise be wasted on millions of duplicate copies of “Sgt. Pepper’s Lonely Hearts Club Band.”
The other big advantage of iTunes Match is that matched tracks are provided to you at 256 Kbps AAC, in a DRM-free format. If you ripped much of your music from CD many years ago, it may be in 128 Kbps MP3 format or worse, so the iTunes Match versions of the songs may be of noticeably higher sound quality. Assuming that you’ll be able to keep these higher quality versions even if you allow your iTunes Match subscription to lapse in a year, $25 isn’t a bad price to pay for not having to re-rip numerous old CDs into modern encoding formats.
Once your library is either matched or uploaded, you can stream your music to your iTunes-authorized Macs running iTunes 10.5.1 or to your iOS devices running iOS 5.0.1. (On an iOS device, just turn on Settings > Music > iTunes Match.) iTunes 10.5.1 itself requires only Mac OS X 10.5 Leopard or later on a PowerPC- or Intel-based Mac, making it significantly more backwards-compatible than iCloud, which is available only for 10.7.2 Lion. iTunes 10.5.1 also runs on Windows XP SP2, Windows Vista, and Windows 7, which could be welcome for accessing your music library at work.
There are some. First, if you have more than 25,000 songs in your iTunes library that were not purchased from the iTunes Store, iTunes Match won’t let you sign up at all (presumably you can fool it by creating a slimmed-down library). Second, iTunes Match won’t upload songs that are over 200 MB in size or that are encoded as AAC or MP3 with a bit rate lower than 96 Kbps. Third, songs in ALAC, WAV, or AIFF formats will be transcoded to temporary AAC 256 Kbps files before being uploaded, but the originals will remain untouched. All other unmatched content will be uploaded as is. Fourth and finally, DRM-shackled songs purchased outside the U.S. iTunes Store will not be matched or uploaded.
iTunes 10.5.1 is a 102 MB download; it’s not yet appearing in Software Update for me, and the Download link on is currently incorrect. However, you can download it from the  page, and it will undoubtedly appear in Software Update shortly.