That other reviewer should actually play the game before making a review.
You can play free mode and a weird little history mode, where you unlock up to 15 songs (i don't remember the amount). However, it IS very cheezy with its jokes.
The gameplay comes from the easy to the very hard, and the songs are quite good, they're lyric-less, though.
The characters you get are 4 stereotypes, it doesn't really matters who you choose, really, you won't really get the time to look at them while you play.
Alternatively from the guitar you can use the drumset, it can be very hard to play with it, you can use A, B, X, Y or L for one drum and the control pad or the R button for the other, in the case of the cymbals you must press both drums at the same time. You can also use the touchscreen to do all the moves.
In my experience, Guitar Rock Band for the DSi is a decent game, and is a good time waster.
....The one thing that bugs me is that you need to unlock every song for every level with the history mode.