Well, GH4 looks actually fully featured. Yeah, they're not going to let you make notes based on mp3s - you have Frets on Fire for that - but I'm sure people will do some cool things since it looks like you can basically do something similar to "Jam Sessions" for the DS.
Of course, the lack of vocals will make the songs kind of suck... but if people can port over some cool instrumental songs - like Canon Rock - I'll be happy.
Also, tried the new songs. Pretty fun, if not a bit hard. I guess that's expected of metal or rock or whatever they are.