The story and acting are so good Brutal Legend should win a movie of the year award. This game is a masterpiece.
I played on the hardest difficulty and there were 3 or so missions where I lost 3 or 4 times in a row and got frustrated. The losses didn't feel cheap (except for the side-mission delivering beer, that sucked) I just felt completely overwhelmed. However I always learned, and I did better each time. On the 4th or 5th try I won, and it felt like a valid victory. If you are having trouble with the huge battles and disliking them you're doing something wrong, that's all I can say.
Just play it. Buy it. Support the developers. I read a lot of "meh" reviews and I honestly don't understand it. If you are interested in the game and the setting you WILL NOT be disappointed. I swear it.