Sorry to resurrect such an old thread (yay for zombie threads!), but I've been experimenting a bit recently with my system and thought I'd chime in with some of my opinion-based observations on 5.1 into 7.1:
My system has been 5.1 since the start. I use towers in all positions (the four corners are essentially identical, three-ways with a 12" woofer, but the center is a smaller tower of a completely different brand, which no doubt causes timbre-matching issues). I use these speakers because they sound good, handle the power I push through them, and they are paid for (the four corners are all about 30 years old now - replaced the rubber surrounds once already). They aren't anything fancy, but everyone likes the sound when we watch movies, commenting on how smooth and natural it seems, not harsh or boomy, etc. I have dual Epik Empires up front, so I don't rely on the speakers for anything below 50 or 60Hz.
I recognize they won't last forever, and perhaps it is time to start researching upgrades. If I want 5.1 I could go with towers all around, but if I want to go 7.1 then bookshelf speakers might be a better choice - I can mount the rear surrounds up high behind the third row of seating. I would prefer to have identical speakers all around, including the center, which is very possible with bookshelves. So as an experiment, I hooked up a spare pair of Sony bookshelf speakers as rear surrounds. (I use these same speakers in a 5.1 set-up in the living room, and they are quite good value, IMHO). I did some basic level matching and EQing, but did not bother running Audyssey on my Onkyo 808. I may do that this weekend, but I have found I don't like the results Audyssey gives - the sound is too "muffled" for lack of a better word.
I had a chance to do some listening last night. Played the first couple of track from Opeth's Blackwater Park 5.1 disk (DTS track) and the first four or five songs from Neil Young's Harvest. I tried out PL IIx Music and Movie, and compared to straight multichannel audio. I have two possible Main Listening Positions in the room - center of front row or center of middle row. In both spots, the non-matrixed 5.1 multichannel presentation sounded as expected, but when I engaged either PL IIx mode it seemed to be a little more "enveloping" from the rear. While there were differences in Music vs Movie, I didn't feel either was bad - it might depend on the source material more than anything. I'll do some jazz or classical this weekend, and throw in a bunch of Steven Wilson / Porcupine Tree, of course.
In case anyone is curious or wants to comment: my two current faves for investigating further for possible speakers are the Klipsch line (which I have heard RF-82s in person and did not find them harsh) and the HSU bookshelves - both horn-based, but very different it seems. Ask me again in a week, and it might be two other brands. So I'll be trolling the speaker forum for a while, and whenever the day comes when I feel the need to replace my current hardware, I may actually be prepared to make a decision...
Anyway, just a light ramble on some recent observations, with some context to help explain. More comments will likely follow.