Well it has arrived, I'm liking what I've seen so far, and it is impressive how it adjusts the difficulty when it think you are getting the hang of it.
Most songs have got 2 options, single note or chords, and it monitors how well you play, and then adds in more and more notes / chords.
The criticisms I have are that it keeps wanting you to check your guitar is in tune about every 5 mins - which is a bit irritating. And when on the chord mode, it introduces more chords on the fly, but sometimes if you don't know (or can't remember) what F7 is for example, it takes a few times of seeing it come up to work out your finger positions, as they aren't on screen for that long.
I've probably only put in about 90 mins since I've got it, but can see me using it, its nice to play along with the rest of the instruments, it also gives you points for timing, and monitors your accuracy.
How it actually works is beyond me, but it knows when you play bum notes!