The material on this set is very interesting, and much of it is well worth having, but I speak that as a hardcore Beach Boys fan-- to a more casual fan, this is really extraneous.
Of the 57 tracks on here, 17 of them are spoken pieces, interviews mostly, culled from the archives. While they're interesting and usually sentimental, there's nothing really revelatory in this.
Beyond that, there's a bunch of studio recording session excerpts ("Salt Lake City", "Wish That He Could Stay", "Carol K"/"Little Girl", "Barbara Ann", and "Good Vibrations"). These are all interesting, but if you're not intrigued by the construction process, this will seem largely extraneous. The "Carol K" one I particularly enjoyed, but "Little Girl" is one of my favorite Beach Boys tunes, so this is no surprise.
Additionally, there's several tracks that are isolated backing track-- songs without the vocals, Stack-o-Tracks style-- "Surfin' USA", "Fun, Fun, Fun", "Good to My Baby", "Be With Me", and "Sail On Sailor"). Again, these are quite interesting, but only to someone more interested in the process of the creation of the songs.
Matching a bunch of backing tracks are a bunch of vocal only tracks-- these might be more exciting to the casual fan as we get to hear those great harmonies without any instruments. On some of them, particularly "Kiss Me Baby" and "Forever", there were vocal parts I had never previously identified that were masked by or doubling instruments. There's also a capella mixes for "Can't Wait too Long" (a brilliant short edit), "I Went to Sleep" and "Add Some Music". And two Party tracks, "Barbara Ann" and "Devoted to You" are presented without overdubs.
With the exception of what I'll discuss below, the rest of the material is alternate/extended versions, much of this ("Little Girl", "Time to Get Alone") is really nice and I'm glad to have them, but again, not anything thats exciting.
The exciting material is the couple of new songs presented here-- "You're With Me Tonight" (mixed from previously unused recordings), "Lonely Days", and a great unreleased Dennis gem called "A Time to Live in Dreams". These three alone make this set worth its weight in gold to any fan, but to the more casual listener, and even to less devoted fans, this is really quite unnecessary.