The heft and presumed comprehensiveness of a boxed set confers legitimacy in some folks' eyes, but honestly, anybody fanatical enough to buy a multi-disc collection probably has most of the enticing "rare tracks" already. And for a new fan, 50 bucks (or more!) is gonna seem a little pricey.
There are two versions of Soundtrack to the Apocalypse: One is three CDs and a live DVD; the other, a limited edition, adds an additional live disc. The box consists mostly of chronological album tracks from the band's American Recordings tenure. Disc two also offers a few soundtrack cuts and rarities (including Slayer's take on "In-A-Gadda-Da-Vida," from the Less Than Zero soundtrack). More important, though, it makes a great argument for the band's 1990s output. The Diabolus in Musica selections are particularly choice -- the grimy, hardcore-influenced guitar sound on "Bitter Peace," "Stain of Mind," and "Death's Head" is frighteningly intense. Disc three compiles rarities, some of which (home demos for South of Heaven tracks) are fascinating and some of which (early live cuts) are just kind of amusing, like finding an old 'zine in the attic. The live material, both on the DVD and the limited-edition disc five, is crushing, of course, but it still pales in comparison to the band's ultimate live album, Decade of Aggression.
Slayer really isn't a boxed-set kind of band. It doesn't need the kind of "legitimacy" that such a career retrospective confers. All Slayer really needs is what it's already got: the respect of millions of fans, the ability to crush any stage it steps onto, and reserved seating in the darkest level of hell.