I'd passed on this initially because of the price, but everyone kept raving how good it was and I sorta regretted it. (I usually either get something the first week, when it's on sale, or wait until the price drops a couple of years later.) You can never have too much Zeppelin.
That reminds me: I got the Complete Studio Masters boxed set ages ago, to get all the albums plus the extended Coda (which, for some bizarre reason, has never been released, making "Hey Hey What Can I Do" only available on boxed sets). Coda's got the four non-album tracks from the two other boxed sets (making this set close to the same thing as getting the other two, only it has the album art and original running order, etc).
Only drawback (if you can go so far as calling it that) is that the first (IIRC) box has "Moby Dick" and "Bonzo's Montreaux" remixed into one medley. I'd taped it from a friend's copy of the box ages ago to complete my collection, but I should check iTunes or Amazon and see if they've got it digitally. If not, check less "reputable" sources.
I've got to wonder why we've never seen any unreleased Zeppelin tracks aside from the four (and HHWCID doesn't really count, as it was a B-side... two of the other songs were recorded live and the fourth was recorded and misplaced before they'd adopted the name "Led Zeppelin," so maybe technically none of them truly count). (Then again, I guess Coda sort of was unreleased tracks that were getting released.) Were they one of those "burn the masters once the album's done" and there simply wasn't anything left? You know the entire frickin' world would shit itself if there was another album's worth of Zeppelin stuff, even if the members had to go back in and polish it up or something.