You have to give Vicarious Visions some credit.  Bringing the Guitar Hero experience to the portable DS was not an easy accomplishment.  The developers at Vicarious Visions' solution to the problem comes in the form of a plug-in fret controller that fits into the DS system's GameBoy cartridge port.  Innovative, yes, but not very ergonomic.  Playing more than three songs in a row will put you in imminent danger of developing chronic carpal tunnel syndrome.

The problem is in the way your DS needs to be held to play the game.  The DS is held sideways in the palm of your left hand with your fingers bent at a 90 degree angle and resting on the fret buttons.  You also need to keep the spine of the DS braced against your palm to keep it from moving too much as you push the fret buttons.  In most playing positions, your wrist will be bent to keep the DS oriented upright, or you need to twist your head while playing to get a good look at the screen.  If you don't think that this would be painful, trying playing two or three songs this way and then get back to me.  Searing wrist pain and kinks in various parts of your body occur with every play session.  About the only way to play without too much discomfort is with your arm flat on a high desk (or on a high countertop while standing) and with your wrist kept perfectly straight.  Which, of course, means that the game isn't really that portable after all. 

If you've played Guitar Hero before, then the DS version of the game will be pretty familiar to you.  The strum bar has been replaced by an image of a guitar on the touch screen.  To strum the strings you use the included guitar pick stylus and move it across the strings.  The top, or in this case left, screen of the DS is used to display the music track and as each note reaches the bottom of the screen you need to hold the corresponding fret button(s) and move the stylus across the touch screen to play the notes.  Unlike the console versions of the game there is no fifth 'orange' fret button, so hard and expert modes challenge you by increasing the speed and frequency of the notes.  The higher difficulty levels are made all the more challenging by the fact that furious fret work will shake around the DS and can even cause the fret controller to work its way out of the cartridge slot.  Star power is engaged not by shaking your DS, but by shouting at it, although the microphone is so sensitive you can just blow hard or rely on a passing truck to set it off.

Guitar Hero: On Tour main mode mimics that of its console cousins, but in the most basic, no-frills way.  You play a tier of four songs, which unlocks the next tier of four songs, and so on until you've unlock all 26 tracks in the game and made them available for random play.  Rather than go with the classic and popular tracks found in other Guitar Hero games, On Tour features an almost entirely new track list.  It seems that the best songs have been saved for use with the console versions of the game, because outside of a few interesting tracks the song list is relatively weak.  The inclusion of pop rock tracks from groups such as No Doubt, Smash Mouth, and Maroon 5 may be an attempt to make the game more appealing to the masses, but all they really do is add some very long and repetitive songs to the game that you'll probably just play through once because you have to.  The size limitations of the cartridge means that there are no bonus tracks to unlock, but given this limitation it's pretty surprising that the quality of the tracks are as good as they are in the game.

Multiplayer is supported locally in several modes.  You can play competitively in a mode similar to the Guitar Hero III battle mode, sending attacks to the other player such as setting their guitar on fire.  You can also play in face-off mode in which you alternate playing riffs or play the same notes head-to-head.  There's also support for co-op play in which one player takes lead guitar and other rhythm or bass.  All of these modes are limited by the same ergonomic issues that hurt the single player play.

In The End, This Game Hath Been Rated: 68%. Even if you can play through the pain, tapping a screen just isn't as much fun as rocking out with a fake plastic guitar.