They did an entire backing track disc of Pet Sounds? I'd love to hear that! I never did get the box set, as it was to expensive.
So I'm going to spend $145 on another multi disc set, anyway. Makes perfect logical sense, huh?

You can buy the PS Sessions box as MP3s for £25 now, or second-hand for under £20. It's no longer the essential purchase it was at first, because the stereo mixes are now widely available, but it's well worth getting at those prices.
And yes, it included stereo backing tracks of every song (except the two instrumentals, where it included "track minus lead instrument" versions) as well as a capella versions of all of the songs with vocals, and lots of very interesting alternate versions.