If you want a complete overview of the scene circa late-1970s to mid-90s. Many of the cross-references and entries reduce or eliminate the pre-1991 coverage in the 5th ed. This made room for what can be seen here, perhaps unwittingly, as the death of the alternative scene, as Option magazine folded since nobody knew what the adjective meant anymore. Stone Temple Pilots, Metallica, Madonna, Prince, Run-DMC are all here, which isn't a recommendation in my opinion. Still, faced with the lack of other works (Dave Thompson's Alt Rock guide and Christgau's decade-length digests are far more, well, individually weighted, for better or worse), this is the best in-print source. All-Music Guide, now having migrated like TP to the web, serves as an excellent counterpart to TPRG's 4th and 5th eds.
I often compare reviews from AMG and TPRG, or check data for references. Between the two, they fill a gap left by Rolling Stone since the 1980s and Spin since, well, then, for the American rock aficionado who can't always find Q or Mojo. TPRG started as an Anglophonic rock 'zine in the middle of the 70s, so its tendencies tilt that direction in much of its coverage. The styles remind me of the heyday of Option or Alternative Press in its array of critics, who often wrote in these magazines. The critics show their smarts and are fun to browse.
Reviewers do often go easier on the music than Thompson or Christgau, but their emphasis on what the more mainstream or over-eclectic critics miss fills the gaps for serious fans. If you use this in conjunction with AMG and your own gut reactions, you'll gain a better sense of what's worthwhile to keep hearing.