Tapping buttons to the rhythm of music games has become increasingly popular over the past decade, fuelled by in-game soundtracks that only make you wish that it were possible to play with your own tunes. Despite paying for an entire game, you end up wading through a sea of mediocre songs to play but a handful of preferred tracks.

Phase still forces you to sift through dozens of tracks, but the problem isn't finding something good. The innovative ability to jam to your own iPod music gives rise to an all-new dilemma that demands choosing favorites from an approved collection.

Phase has you tapping colored notes as they stream down the screen on three separate tracks. Much like the mobile version of Guitar Hero III, it's your responsibility to keep the groove going by carefully timing presses of the clickwheel and center button to coincide with the moment notes pass through targets at the bottom of the screen. In addition, sweeps have you sliding your finger along the clickwheel to grab points lined up on the track. Nailing continuous strings of notes and sweeps enables you to clear checkpoints, as well as boost your streak multiplier for extra points.

Each song is divided into a series of seven checkpoints. Successfully passing through a checkpoint requires earning a certain number of stars awarded based on your performance. Naturally, earning stars means hitting as many notes as possible. Fail to acquire enough stars before passing through a checkpoint and you lose a heart for each star you're short of the goal. For example, if three stars are needed to clear a checkpoint and you only have one, you lose two hearts. As it goes, if your hearts dwindle down to nothing your game ends.

A short soundtrack of seven songs comes included with the game, but the real attraction is in the ability to add songs from your personal library to a special Phase playlist in iTunes to tap and sweep through your own music in the game. Quick Spin mode lets you select a single song by artist, album, or song title. Alternately, Marathon strings together a minimum of five tracks into a single session. With each song you clear, the difficult rises until you drop out of the game.

The concept of utilizing personal music for gameplay isn't new – The Sims DJ and Musika both do so to largely great effect. Phase differs, however, in the unique way in which it takes your music and delivers a fresh rhythmic experience every time you play. Note patterns generate dynamically for each song, so you're guaranteed a different experience for each song. It's a revolutionary step forward for music gaming and one that taps into the strengths of iPod.

The innovation in Phase doesn't come without a bit of a stumble, sadly, in that note patterns don't always stay on the beat. When playing a personal track, you'll notice that notes are occasionally placed at odd intervals. It's obviously an unfortunate byproduct of the game's dynamic design, but that doesn't make it any less aggravating. Tackling a song on any of the higher difficulty levels becomes quite frustrating when notes begin to stream down the screen irrespective of the tempo. Sure, part of the challenge of any music-based game comes in the form of notes that play around with the rhythm of the song, but it feels less playful and more erroneous in this case.

Speaking of difficulty, there are three levels available by default: Easy, Normal, and Hard. As you'd expect, note patterns increase in complexity the more advanced the level. The requirements for clearing checkpoints also grow more difficult depending on the level. On Hard, you'll need to acquire upwards of three stars to pass a checkpoint unscathed, which is obviously much more difficult to gain than the two needed on Normal.

Ultimately, near-limitless value can be squeezed out of Phase thanks to its inventive use of music stored on your iPod. In fact, all of the game's great appeal comes as a direct result of this feature. Problematic note patterns have the game missing a beat, but on the whole it's and experience that's definitely worth tuning into.