They used them. They did not use them in the pattern or style that became common in Thrash Metal. Double Bass has been around for a long time. The pulsing driving constant pound of them though is said to have first occurred on the song Overkill by Motorhead. He did not get the double bass to do that, he was just trying out the new stuff and the rest of the band heard that particular pattern and decided to write a song to it. Obviously it had been around as an instrument for far longer than that as that is not even why Motorhead had the double bass themselves.

Start that video from about 4:22 and it talks about the double bass... this is actually Sam Dunn interviewing him and it is in that video that I first learned this information.

Metallica as guitarists were better overall than MOST people playing at the time they played. I of course didn't start playing myself until I was 16 or 17, so I was already into Metallica before I played guitar.

Looking back now I think Kirk Hammett is pretty good, but I wouldn't call him great. There are so many people that swamp him in terms of skill. He does get some nice use out of that Wha pedal though. :)

Dime definitely was a bad ass. I really like John Petrucci a lot too... that guy is phenomenal and very disciplined... practices 6+ hours a day to this day and has since he was in high school. I'd say there are few better in terms of skill.... there are people that are better that have that sort of magic spark that can't be taught or learned like Hendrix and Dime seemed to have had.

Yet if you want to go on the top notch level you can get out of sheer practice and study then I put Petrucci at the top there. He stands along those like Vai and Satriani.