I dug the streaming Atmos enough to pick this up when Amazon had it for ~$85 earlier this week. The box set is pretty unimpressive - they could easily have fit all of this (sans maybe the poster) into a Jethro Tull/Marillion-style book or maybe even one of those three-panel digipaks the ELP reissues used - but I'm pleasantly surprised by the dedicated 5.1 mix. It fills the room nicely with the lead vocal upfront, guitars in the phantom sides (it's interesting to watch this on my SH-3433 scope, sometimes the guitar on the left is slightly biased to the front and the other towards the rear), drums seemingly everywhere, and some rather effective room ambience plus the odd isolated element filling out most of the back soundstage. I would definitely reject the notion that the 5.1 mix isn't 'discrete' though, the backing vocals in "Long Time Running" and "Three Pistols" completely vanish if you mute the rear channels. The Atmos mix actually seems more front-centric to me!
Thanks for sharing your thoughts as I decided to pass on this because of what I heard about the surround mix (that it's not very discrete). May have to reconsider it now.