There was a black Friday special for Guitar Hero World Tour for the DS so I decided to give it a shot and picked it up for $29. It comes with an attachment that inserts into the GBA cartridge holder in the front and lays against the bottom of the device. You place your hand into the attachment through a hand loop so your fingers can hover over the four colored buttons. The object is to hold down the buttons (frets) while strumming the touch screen with your pick-shaped stylus. It definitely takes some getting used to and (when played in the privacy of your home) can be quite fun. The songs are Guitar Hero quality and the graphics are fun and helpful.
There are a few problems though. First, I can't imagine playing this thing in public. Whether it be on a park bench, in an office, on the subway, or wherever, there is no way to appear normal when you're jamming on this thing. You're certain to get odd looks which, for some people, may be a plus. Secondly, ow, ow, ow. This thing cramps a hand like no one's business. Admittedly, I have huge palms and this is a very small device, but even my wife -- whose hands are like that of china dolls -- had issues with this. These two things combined really take me out of the playing experience.
I'd recommend GH: World Tour only if you're a huge GH fan with average sized hands who doesn't look like a fool when playing this in public. For everyone else, just pick up Elite Beat Agents.