I know I'll dissonate to the majority of reviewers here.
Yes, this is one of the 1st documented thrash-metal founding records. Yes, when I first heard it back in 1989, I was immensely impressed. Yes this is the raw sound, unique in a way.
But as time flew by, now, 20 years since its release, initial impression has faded away, and in my eyes it's not as interesting as it seemed then. Main reasons for this fact are (1) All band members do not sound very proffessional here yet, and would dramatically improve their performance even the following year, (2) this was just a template for brutal satanic thrash, that would later be developed into full glory in more deserving records, and (3) most of the songs here have countless live versions which are by far better than here in the studio.
Thus, recognizing the importance of this album, I give it only 3 stars. The real Slayer started the next year, with "Haunting the Chapel" EP.