At the end of the day, Guitar Hero: Metallica is essentially what it sounds like: a Guitar Hero game for Metallica fans; by the fans, for the fans in the purest sense. The set list is excellent, especially if you're a metal fan and most of the songs are very pleasing to play. In addition, playing as any member of the band in the locales is bound to bring fans into the experience even further.
Unfortunately, some odd design choices are ultimately what keep this game from feeling like a complete package. This is a great game, given you're a fan of the band or genre. On the other hand, if Metallica or metal just don't cater to your tastes, then you'll want to look elsewhere.