The on-wall M22s have been growing on me. I think the major differences in comparison to my bookshelf M22s are the tweeter and crossover, though. My bookshelf M22s are seven years old (wow!) so a number of improvements have been made to the design since then. Even though I know I'm losing out on some bottom end with the on-walls, I think the other improvements more than make up for it. With an EP500 in the mix to take care of that bottom end, I'm pretty satisfied with the sound.
This is a good thing, since my wife likes the aesthetics of the on-walls about 300% more than the bookshelves.