I agree with the people saying it won't be for everybody, but neither will Aerosmith. When they do a single band game, the people who are not into that band won't buy it. By that same token, i wouldn't buy GH: The Eagles, or REO Speedwagon. Not that they are not good bands, but if I am going to be playing GH, i want to be playing kick ass guitar. Metallica has kick ass guitar, any way you cut it.
About the kill em all splits, i think you might have a point with the masters not being available .... That would sadden me, however, i would assume that metallica would probably re-record tracks from kill em all like the sex pistols on GH3.