Apple's iTunes Store is convenient, cheap and cool. But its music downloads can come with nagging worries driven by the copy-control software behind the iTunes Store — which generally limits playback to Apple's iTunes software and iPod music players.

What if the iPod isn't the best music player two years from now?

What if I get a computer that doesn't run Windows or Mac OS X?

Late last month, Apple began offering an answer to those anxieties: "iTunes Plus" songs and music videos, free of usage limits, that work with other companies' products.

It's not that the restrictions of iTunes purchases are so oppressive — Apple hasn't sold more than 2.5 billion songs on iTunes by accident.

But it's easy to think of ways that an iTunes purchase can lack the utility of a plain old CD. For example, you may love your iPod, but what about playing iTunes purchases on a Treo or BlackBerry?

The usual remedy for that scenario — burning a song to a CD, then copying that same CD back to a computer — takes time and loses some quality. An iTunes Plus purchase represents a far more elegant solution.

This option does have downsides. ITunes Plus downloads cost a little more and, so far, are an option for only about 300,000 songs out of the more than 5 million carried on the U.S. version of the iTunes Store. The files also occupy about twice as much space as standard downloads.

But a few days of testing iTunes Plus made one thing clear: Those drawbacks are a small price in exchange for ending the angst of buying music tied to one firm's products.

Buying iTunes Plus music requires installing the latest version of iTunes, 7.2 (Win 2000 or newer, Mac OS X 10.3.9 or newer) and clicking on the "iTunes Plus" link on the iTunes home page.

Individual songs go for $1.29 each, 30 cents more than ordinary iTunes downloads, but most albums and music videos cost the same either way.

You can also download iTunes Plus copies of older purchases by clicking an "Upgrade My Library" button in iTunes. That upgrade costs 30 cents a song, 60 cents for a music video and 30 percent of an album's original price for a brand-new download of each.

(Movies on iTunes, which come with far stricter use limits than music, aren't available in iTunes Plus.)

The immediate benefit of that expense is no longer having to count computers. Instead of being limited to playing an iTunes Store download on five machines at once, you can copy at will.

You could even give a song to a friend. But Plus downloads, like all iTunes purchases, come stamped with the name and e-mail address of your iTunes Store account. So if you share one online, the music industry's lawyers can know who you are.

Here's a better reason not to upload an iTunes Plus song to a file-sharing service: Don't be a jerk. This is the fairest download option a major music label has yet offered.

The second-worst problem with iTunes Plus is the larger size of the files, turning a 1,000-song iPod into a 500-song device. But as disk space keeps getting cheaper, this issue will wither.

The worst problem with iTunes Plus is the scant selection of tracks. All come from one record label, EMI. Apple has invited others to join the venture (many independent labels seem inclined to do so), but none have shown up in iTunes Plus.

That leaves EMI with a substantial advantage over its competitors. Until now, there hasn't been much of a reason for people to prefer music from one major label over another. Now there is. EMI's rivals need to take the hint — soon.