I had my tested in 7.1 mode on a Meridian processor. It was MG-IIIa mains, MGCC1 center and MG10.1 surrounds.
Can't say too much about movies (I mean, do I get 7.1 just to get a better flyover?) But it was definitely improved, with the Meridian unit synthesize the 2 extra channels. Flyover was more continuous, dopey demoes that went around all speakers was also more continuous.
With music surround modes, it shined. The side hall sound became more prominent. Ambisonic's 1st reflection became more prominent. Soundstage expanded seamlessly beyond the main speakers when it existed.
As far as placement, it was a pain for me. One speaker with only the top half above the sofa's back, and the other one sort of next to a door. Definitely a asymetrical placement in which I needed to specify different distances to the L & R side surround.
I also played around with turning it sideways so that it simulate a THX dipole. The sound became less localizable, and the sound came more from the rear. But Meridian don't recommend that for their music mode so I didn't bother with it for too long.
At the end, I put my setup back. The maggie sides took too much space, plus I had to scrounge around for extra channel amplifications. Maybe if I get a 5ch amp to complement my 2ch front, I will think about it again.