(Since you guys can look up my cheevos, I will be straight with you. I have finished the Solo guitar career on Hard. Not that I can't do Expert, I just always do Hard first to warm up. I have not done any of the other instruments, nor have I done online, because when I buy a Guitar Hero game I want it for one reason, to play plastic guitar by myself.)
One day in high school, freshman year, one of my friends handed me Master of Puppets and asked me if I would give it back to another one of my friends for him. I said OK, and when I didn't see the second friend for the rest of the day, I took the CD back home with me. I had never listened to Metallica before, even though I obviously knew who they were, so there I listened to it from start to end, and fell in love. Even though I gave my friend his album back the next day, I immediately went out and bought as much Metallica as I could.
I will go ahead and state at this point that I like all of Metallica up until St. Anger. Most people don't like anything Load-to-present, and some people don't even like anything Black Album-to-present. But I liked Load, Reload, S&M, and Garage, Inc, so that might bias my thoughts of the game.
GH: Metallica is the first GH I have played/purchased since Guitar Hero 3. I saw what Activision was doing with the difficulty of the series (going from "Gee these note charts are challenging." to "What the fuck, these note charts don't even make sense...there's not even goddamn music where those notes were!") and I wasn't very happy about it. I switch over to Rock Band and never looked back.
I have to say though, Activision surprised me with GH:M. I heard that GH: World Tour wasn't much prettier than GH3, so I was prepared to deal with a sub par game as long as it was to Metallica tracks. But the note charts aren't ridiculous, I mean they are challenging, but these are Metallica songs we're talking about. While they're not unfair, one of the downsides to playing Metallica songs is that your hands will get tired very quickly, since most of their songs are 8+ minute epics.
On the flip side of having such long songs, career mode is not long by any means. Since it no longer matters how many songs you beat, just how many stars you get, every tier except for the last one can be as short as 2 songs as long as you 5-star everything. Which means you can feasibly beat career mode without touching 4/5 of the game's content. But then, I guess that's what Quickplay is for.
I know everyone likes to talk about how Metallica hasn't been good since 1990, but a vast majority of this game is early Metallica, so that shouldn't stop anyone from picking this up. In fact, there are only 2 2000-present songs, so it's really not a problem.
The only real negatives I can think of is the layout of the display (Star power, multiplier, note count, etc.). Once you get used to the Rock Band layout, it is hard to go back to something that isn't as efficient, but it is by no means a dealbreaker. Also I would have liked the option to play the Metallica's covers of the originals from Garage Inc. (Tuesday's Gone, Am I Evil, Turn the Page, Stone Cold Crazy), but I guess you can't have everything.
I would have to say if you are any kind of Metallica fan, GH:M is worth it.