Unlike most of the music games on the iPhone, you can actually use the multitouch screen to "strum," in a minimal imitation of the game's guitar controller. The game also offers up customizable rockers, and it's Facebook-connected as well. The soundtrack is pretty impressive, too, although the pickings are slim without in-app purchases. There are six songs to play right away, from Queen to Rolling Stones to Weezer, and then you can buy various 3-packs of songs from different bands like Protest the Hero and Vampire Weekend.
The app itself is priced at just US$2.99, so it looks like Activision is aiming to compete with the $6.99 Rock Band by going cheap and making it up on the in-app purchases. Which is actually not a bad idea -- this way, you only have to pay for the songs that you want rather than a big song list that you're not interested in.
Otherwise, the app seems like a respectable version of the console game on Apple's handheld. It's a little late, given that competitors have all released their own versions already, but it's priced and featured accordingly. If you've still got room for another music game on your iPhone, it's worth checking out.