I'm an on-wall owner and I absolutely love them. I use them as mains with a a Protege center. I haven't compared them to standard Emeralds, but going in I was expecting to lose some imaging and soundstage due to their on-wall nature. I haven't noticed anything to complain about in those departments, even when used just for 2 channel.
Since we're acting like Mike isn't around, I'll say that one aspect of the on-walls that is underhyped on their web site is the bass output of the on-walls. The spec says 65Hz, but I'm getting solid output that according to my measurements doesn't fade until below 50Hz. There is some real magic to the port design here -- they get a nice kick off of the wall.
I'll probably move to a pair of Protege V mains one day for uniformity across the front. But, until then, I'm very pleased with the on-walls and I emphatically recommend them.