So, I finally picked up Guitar Hero: On Tour for the DS. It's fun, but as I figured, I suck at it and my hands are so insanely huge I can't get a decent grip on the grip.
But, aside from that, I still like to try and play it - but here's something I discovered yesterday. If you attempt to use the Guitar Grip with the official Nintendo DS Headset (the mono earpiece with the mic - I'm guessing the third party ones are similar) - you're screwed. The Guitar Grip blocks the mic port on the DS Lite. Works fine with the original DS though.