Yes, but long ago, mine and others (used to work for a dealer and helped install systems). Diffusors generally cost much more and must be very large to affect low frequencies. My present treatment is all absorption; I hope to gradually add diffusion, probably replacing some of the absorbers.
What do you want to know? Diffusion tends to help keep the more "spacious" sound many like while still breaking up higher-frequency cancellations. I have also heard a system with large (8' tall, roughly 4' wide and 4' deep) corner diffusors behind them, and they helped quite a bit. IME, diffusion does not provide as stable an image but tends to "open up" the sound more, and most people prefer the sound with diffusion rather than absorption. A room as dead as my current setup (the smallest and deadest room I have had them in, I think) provides excellent imaging and virtually no cancellation effects across the listening position, but all ambiance comes from the recording and not the room.
With the exception of the one system with massive diffusors, bass was better with absorbers behind the Maggies. The wavelengths are such that diffusors have to be huge, and even absorption at LF is difficult to achieve in practice. I have 10" corner traps and numerous 6" and 4" panels, all with 2" - 4" air gaps, along the front wall. A large room that allowed the Maggies to sit well out from the back wall is also less sensitive to that durn back wave.
All this applies to ESLs as well, natch.
Ethan can cover this in much greater depth than I, as can others who have posted here. My audio work and those pesky grad acoustics classes were long ago...
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley