Right I'll come out with it: £70. £70 is what you pay to get the game and a nifty controller. But I think it's worth it and here is why.
After setting up your guitar you can navigate the Wii menu with the joystick and green button. Anyway on the menu you use the strum bar to move up and down, and different buttons to, select, go back etc. When you load a song in quickplay or career, a guitar neck shows up and the difficulty depends how fast the notes come and how many strings you use. For example: Easy, 3 strings, quite slow; Expert: Insanely fast, all 5 strings. There are three types of notes just to really get you confused. Long, Short and Chords. Long notes are notes you have to hold, short notes you only have to hit, and chords can be long or short and consist of two strings.... or more.
As for the relevance from the notes to the song? Well actually quite good. It sticks to the beat and if you miss a note, your Wii remote yells at you and the guitar element of the song stops until you hit the notes again. For a £70 game, it's worth it. But take my advice: shop around - after a little while I found it for £55 in HMV.

+ Nice controller
+ Good Song List
+ Good Note - Song relationship
+ ALOT Of offers around
- £70