Of course, when something like this is ported to a handheld, some things get cut (like one of the controller buttons for example), but I don't think you'll be disappointed with the result. The game is still the same, fun, Guitar Hero that you've grown to love, and now you can play it on the go.
The Guitar Hero controller attachment plugs into Slot 2 (the GameBoy Advance cart) slot of your DS. Sorry, if you own a DSi, you don't have a slot for the Guitar Hero controller to plug into. Then, you put your hand into the strap on the controller which lets you hold the DS in one hand. That leaves your other hand free to strum the guitar using the included guitar pick stylus on touch screen.
For a limited time, Play-Asia has the Asian version of Guitar Hero: On Tour with the controller attachment on sale for only US$39.90. That's a steal! If you want even more songs, I'd suggest picking up Guitar Hero: On Tour Decades (without the controller attachment). The one controller will work for both games.
I think you'll be hard-pressed to find a negative review of either game. I mean, if you don't like Guitar Hero, that's one thing, but if you do, and you want a portable version, this is it! Here's an ad for it just so you can see what it looks like.