I went from 7.2 to 9.2 and I liked the difference - as has been stated, the size of the soundstage grows and the "surround" in surround sound improves. I first tested with some small satellites I had laying around from years ago, and the effect was pleasant, so I ended up buying some Boston bookshelves to go with the rest of my Boston setup. I'd love to have gone with some towers, but the mouting would have been really difficult. Due to my subwoofers being so tall, I just positioned the bookshlves on top of the subs, one just inside each corner of the room. I often wonder why this wouldn't have been the preferred position - rather than choosing between WIDE speakers and HEIGHT speakers, using a high mounting at the edges of the front wall gives you the chance to due some really neat things in the mix.
I think it's that there's almost no content designed to be played on height speakers
Ehh. DVDs rarely if ever have discrete rear surround channels in the mix, yet layering PLIIx over DD and DTS 5.1 makes a noticeable improvement in my system without just repeating the side surrounds. In the same respect, layering PLIIz over 5.1 or 7.1 sources makes a nice improvement without just repeating a channel.