The first Zeppelin box set made sense. Three unreleased tracks and the best of 9 studio albums of one of the greatest rock bands of all time. But then someone had the idea to release all the rest of Zeppelin on a two-CD set, and the result is just what one would expect from a compilation of leftovers.
This set is useful only if you've purchased the first box set and want everything else the band offered on their albums. Sadly, this strange marketing decision overshadows the music, which is the other side of the story, the 
Lost World to the first set's Jurassic Park, if you need a stupid comparison.
Since Zeppelin was an album-oriented band, taking the minor but important pieces out of context and lumping them together isn't much fun, like an album full of B-sides. You get the experiments, the filler, the fun, the non-epic side of Zeppelin that truly completes the picture.
The first disc starts with the incredible bass drum kicks of "Good Times Bad Times," then veers into the hyper "We're Gonna Groove," the folkish "That's The Way," and the unreleased blues track "Baby Come On Home," which is nothing special. The experiments here are more hit than miss, especially on the honky tonk "Hot Dog" and "Boogie With Stu," which sounds nothing like Zeppelin had ever done. "Carouselambra" closes the disc out, and although it tries hard to be an epic, repetition does not a good song make without hooks.
Sadly, the second disc falls off the tracks, which were not that steady to begin with. "Darlene" and "Walter's Walk" are average, "Hats Off To (Roy) Harper" is strange and annoying, "I Can't Quit You Baby" is standard blues and "Hots On For Nowhere" sounds like a band on autopilot. A few gems are hidden, such as the rollicking "How Many More Times," the funky "The Crunge" and "Bring It On Home," and the disc closes with the epic "Tea Tor One," the slowest and most depressing song the band ever wrote.
The liner notes describe this as "the rest of the best," which is true in one sense, but this has the feel of thrown-together leftovers, and much like reheated pizza, it's not as good as the first time around.