There are two types of box set: those that collect obscure (live) tracks and B-sides, and those that merely collect an artists published work as a single set. Iron Maiden did the former, Black Sabbath did the latter. And Priest? More towards the latter, but with a few bones thrown in, mostly consisting of live versions. However, they did include the excellent live DVD from Memphis, which alone would be worth half the retail price. I like that the tracks are in chronological order, so listening lets you hear the evolution of the band, and in fact metal itself, since this band personifies metal more than any other.
I was a fan from Hell Bent through to Defenders, but I lost interest after Turbo, so both the early and the late tracks in the collection are new to me. Disk 1, 2 and most of 3 are great, and belong in any metal fan's collection. Disk 3 is truly excellent, up to and including Turbo Lover, after which things go rapidly downhill. From disk 4, I like Painkiller, Blood Red Skies, Nightcrawler and Ram it Down. The other Halford stuff from his final albums are weak, I'm afraid, and it's just as well he left when he did. I'm still getting used to the Ripper stuff so I'll hold judgement there.
In additon to this, I recommend Unleashed in the East, since I find that they did their early songs much better live than on the original studio albums. There isn't much overlap between this collection and that one, only Victim of Changes, since the live tracks here are taken mostly from other shows.
In summary, worth it, especially if you need to refamiliarize yourself with the Metal Gods. If you already own the remastered cds, then you're probably hardcore enough to buy this one anyway.