Undeniably influencial group. They are acknowledged as being one of the very few bands who gave birth to metal music. Some of their early albums are now considered to be iconic, if not legendary. Cliff Burton's early death was a terrible loss for Metallica and their fans as he was obviously a tremendous creative force within the band. The band's music definitely changed after Master of Puppets, which was his last album.
It didn't slow their race to fame, however, and they ultimately became extremely popular, especially with the self-titled "black album". They met both critical and financial success in the early 90's, but some of their fans didn't like the slower, more commercial approach the band took at that time.
They released numerous albums on a very regular basis throughout their existence, which unfortunately contributed to seeing them publishing albums that had a slightly lower quality when compared to their early works; the songs being more predictable, the solos uninspired if not almost unexistant ( ex: "St. Anger" ). They also began to have a more corporate approach towards the music industry, which led many of their hardcore fans to turn their backs on them. They thus experienced a decline in the late 90's and early 2000's but remain tremendously famous to this day.
James Hetfield's powerful, gritty voice and Kirk Hammet's solos clearly are the most appealing traits of Metallica's music. As a musician, I believe Lars Ulrich's lack of improvement as a drummer throughout the years has been an unecessary weight on their creative process. The recent addition of Robert Trujillo was like a breath of fresh air for the group, however, and I believe the recent revival of the band with the "Death Magnetic" album is due to his contributions.