iPhone and iPod touch are the latest destination for the Guitar Hero extended world tour, having played practically every other gaming venue under the sun. ATM and petrol station displays are the final stops.

It's an act that's far from fresh - notes have been tapped out on the touchscreen in Tap Tap Revenge and Rock Band for some time - but the extra rehearsal time has benefited Guitar Hero. Despite its lack of features, the core rhythmic gameplay has been orchestrated with enough creativity to make it a worthwhile gig.

Catering to the bass

Of course, the fundamentals remain untouched. As coloured notes stream into the foreground on four vertical frets - guitar or bass, it's the same - it's up to you to keep the beat going by tapping them as they cross the rhythmic threshold. Fumble and you risk botching a song, the music striking nasty chords and fading with each missed note.

Joining basic taps are new strum notes and slides that take advantage of the touchscreen. Strum notes appear as arrows stretched across two frets, the objective being to swipe across the screen in the specified direction. Similarly, slides link regular notes together across several beats. For these notes, you hold a finger to the screen and move it as the notes reach the bottom.

These simple, almost obvious additions are the primary reason Guitar Hero succeeds on iPhone. Swiping and sliding a finger across the screen is convincing substitute for actually strumming a guitar. The game feels more interactive as a result of these new gestures and it's more enjoyable than the repetitive tapping of competing titles.

One man show

The game is off-pitch when it comes to modes of play, unfortunately. Multiplayer is absent. Bluetooth, local wireless, online - there's none to be had. With both Tap Tap Revenge and Rock Band providing compelling multiplayer, Guitar Hero really misses the mark. At the least, a co-operative mode via Bluetooth would fill the gap.

A more significant oversight is the poorly structured single-player experience that forgoes a linear career in favour of a confusing slate of challenges. With all six songs unlocked from the start, your objective lies in completing song - and instrument-specific challenges.

For example, you're asked to achieve a 50-note streak on 'Paint it Black,' from the Rolling Stones or hit Star Power so many times during a song. Complete challenges and you're awarded equipment and clothing with which to customise your rock avatar, male or female.

Riff off

It's a neat concept because you're supposedly motivated to take on challenges at your own pace: however, the tiered nature of the challenges is annoying. For instance, if a bronze challenge asks you to hit ten slide notes in a specific song and the silver demands 15 notes and you manage to get 17 slide notes, only the bronze challenge is marked complete.

Hitting the required number of notes only unlocks the silver challenge - you have to play the song again if you want to actually earn the silver challenge. As such, you're likely to just set the challenges aside and play for fun or placement on the leaderboards.

Activision has clearly made an effort to tailor Guitar Hero to iPhone and iPod touch (although, some optimisation is in order to improve the performance on iPhone 3G) and the gameplay is different enough to be worthwhile. Without a more compelling single-player setup and some form of multiplayer, though, the game's appeal is limited.