Yeah Flight 666 is my fav live album by IM. Bruce is very good but I have to be in the mood for him. The only reason why I don't think IM is the modern day Metallica is because they have been around just about as long and are old. We need a new band to take Metallica's torch at least for the next two decades. Really only Avenged Sevenfold has done it so far because they have gone mainstream and if they can come out with a really good album to rival City of Evil (their best so far), then they will be a good bet to take the throne. Easier said than done without The Rev tho, and to be honest A7X doesn't really have true metal roots. If they did, they would be opening for Metallica like a Machine Head, however I can see the flipside in that MH doesn't want to go mainstream and "sell out" although I think they eventually will go the same route as Metallica. In today's modern era, any time you can get mass appeal it can only help and the diehards will still listen to a band's earlier heavy stuff and proclaim "they used to be good during x era".
If you haven't seen Machine Head live, here are some links to check out:
http://www.youtube.com/watch?v=G-SohL691kc
http://www.youtube.com/watch?v=BDs5ag-mMVc
http://www.youtube.com/watch?v=fSed_FPXWrI&feature=player_detailpage#t=803
http://www.youtube.com/watch?v=xh1DQR7H2KE
Metallica's James Hetfield playing guitar with Machine Head for Aesthetics of Hate live (sound is crappy but funny to see Hetfield jamming with them):
http://www.youtube.com/watch?v=3hqAVTntLco
Dio acoustic cover of Die Young by Rob Flynn (lead singer):
http://www.youtube.com/watch?v=M2DlueFfxuU