I listened to the 5.1 mix all the way through today on the way to a job, and the 4.0 mix on the way back. Both uninterrupted and with an ear for detail.
First off, I somehow expected that I would not like the 5.1 mix, and for the first 5 or 6 minutes, I did not. It sounded like I was listening to a stereo CD and the DPL circuits in my Acura were "doing their thing". However, when "Welcome to the Machine" started, my smile appearred. It was the first point in the 5.1 mix where I could pinpoint instruments, acoustic guitars to be precise. Very nice. Of course, the sound effects were in the back, front, in a good surround mix, but it was in this tune that the music filled the car in a non-ambient way. As I listened all the way through, the pattern of a) Sound Effects = good discrete surround, b) Music = more of an ambient surround appeared. I suppose this is the old "retain the feel for the original stereo mix" stuff, but my POV is this is NOT the stereo mix, it's the surround mix. Aside from "Welcome", the pinpoint surround of the audio was hit or miss. There were a few guitar parts that I could hear in a specific spot, but nothing that went "Wow, check this out".
Then, on the way home, I did the 4.0. Of course, this is quad, and there is a lot of seperation. The opening guitar solo "da-da-da-dum" (or however you want to describe it) was purely in the rears (in the 5.1, it's everywhere). This mix is well known, so describing it is moot, but let me say that in the 4.0, I could pinpoint the vocal of Roger and the vocal of David when they were singing, where in the 5.1 they sounded mixed together. I know that some don't like it this way, but then some do. I enjoy being able to listen to a specific vocal or solo when it's "pulled out" of a mix, giving it more detail.
So, the 5.1, I'd say, is a modern, very fine, surround mix. The 4.0 is more of a "wow" deal. If you want to impress someone with surround, you'll probably play "Welcome to the Machine" from the 5.1, or any tune from the 4.0. The audio quality is "pretty good for a 30+ year old recording. It does sound great, both versions, but it doesn't compare to something new like "Morph the Cat" in detail, then again, it's not the same type of music.
Is it worth $100? Well, I can answer that with this. I paid more than $100 for my Q8 of WYWH, so I guess, to me, it is.
Now, what do all of you guys think?