It's funny, I can see I've rated this a '6' at some point in the past, but I think now I'd probably rate the mix itself as a '7' and my overall enjoyment as an '8'.
I think when you hear a 5.1 mix (or quad mix) of something you're even somewhat familiar with in stereo, it's hard to be entirely objective the first few times you listen to it, because you're battling your own preconceived notions about how you expected the surround mix to sound, and whatever disappointment might be attached to those expectations not being fulfilled.
Despite not being as aggressively mixed as most of us would like, the attention to detail in the mix is excellent in my opinion - little nuances like reverbs are handled superbly, and by Shelly Yakus (who engineered the original album) no less.
I think it also has to be mentioned that sonically, the 5.1 mix is a colossal upgrade on the stereo mix, and worth buying for that reason alone if you like the music at all. In my (humble) opinion, pretty much every BOC stereo mix sounds like hot garbage from a frequency response and dynamics perspective. It's weird, the engineering during recording is obviously good (as evidenced by how this 5.1 mix scrubbed up) but they all sound like they took a really well recorded rock album, played it through an AM radio, and recorded the output on to the master tape - no bass, no treble, all midrange and flat as a pancake dynamically. I guess it must be something to do with some of the gear that those studios were using in the mid-'70s, because the Aerosmith albums from the same period, and Rick Derringer's Spring Fever have that same kind of sound. I was going to suggest the poor sound quality of their albums being one of the reasons they never attained superstar status (I know it's certainly put me off devoting more time to their back catalog) but Aerosmith obviously did fine in spite of a similar sound, so I dunno!