Might chip in on this one...
I've been using these since I first tried at NAMM, worked wonders in our booth.
You could hear the same guitar tone where ever you were in relation to the booth
(was demoing for some pedal endorsers), and just for the fun of it, I asked one of the sound police guys
to walk by while I was playing, and measure the Db. Was kinda cool that the Db was pretty much exactly level
all the way in the vicinity of the booth.
Since then I've used them (have 2 for my 2x12's) on every single gig I've done from Sweden
to China and Japan. I've completely stopped using monitors on stage for anything but vocals,
the dispersion is that great. Not cheap definitely, but worth every penny to me (bought mine from Thomann).
SIngle best investment I've done to my live rig in like forever
Like Jim says, Joe's setup with Plexi "cages" for the cabs is not meant to disperse the tone,
bur rather to block the beam from the audience (those 4x12's tend to be very directional).
ANd as we all know he runs his heads through palmer's and then feed that to the monitors on stage.
I've used that setup exactly before, and it worked well, but I felt it did limit a lot of the tone,
since you need soem really good monitors to give you a even half-decent guitar tone (imo).
I think Joe would be floored by the stage tones he'd get with these, but then there is the problem of
using 4x12s, since you can't "deeflexx" the top two cones. When I use a 4x12 I tend to either put a towel
down over the top two (looks crappy but hey), or if I have a stereo cab, turn it on it's side and just use one side
as a over-sized 2x12, also works beautifully (if your amp isn't to much for the cab to handle then of course.
EDIT:come to think of it, I actually have a live video from one of my clinics in Beijing this October
where some local guitarists came up to jam with me. The guy that sent me this vid was standing pretty much
in front of the second amp that didn't have anything in front. The amps are tonally set pretty much the same,
EQ-wise, and we're using the same OD-pedals (my PRS also have two single coils in neck, mounted
in the humbucker position). What I'm getting at is that most of the tonal differences you hear is from the
Deeflexx and the player, so even though the video is pretty crappy, it's still a fair approx. of the directional effect differences.
http://www.youtube.com/watch?v=NNjgZ19uaLc
EDIT 2: @ mbcl I hear you about reversing the cab, that works surprisingly well with open back cabs in my experience.
Only thing is that you loose a lot high end, but it does save the heads of the audience
one way I haven't really gone the distance with trying is in-ear systems and the amp off stage,
but from what I've tried I really don't dig the tone I hear through the in-ear (might be the quality of the in-ear though).