I had 5.1 setup for about a year and added extra pair of QS8's recently. In many cases they are almost silent (most movies even with DPLIIx engaged send very little to the back). When they do however the effect is very tight. For example one of the final scenes in Spiderman 2 when there are voices from the back is very disctinct. I have second row very close to the back wall and so I went with QS8 and not direct radiating speakers such as M2 or M3. I don't regret - otherwise I think that would be too localized to a speaker just over your ears.
For DVD-Audio and SACD playback (using analogue 7.1 inputs on the reciever) I bridged the inputs for rears and the sides. This pushes surround information to the side-rear pairs and to me sounds much more balanced than when just the sides were playing (my "sides" are slightly to the front of the seating, especially the second row so this bridging brings it more in line with Dolby recommendations).
Axiom Setup 
