I've never ran coil overs before, this is merely what I've read, heard, and been advised through various avenues. Icon, King and Fox all represent the top echelon of race coil over. Icon uses digressive valving whereas King and Fox use progressive. While both have their pro's and con's, when used on a KDSS equipped vehicle it has been recommended to stick with progressive shocks, as the combined effect using digressive may be too stiff for some drivers.
There is one website in particular that has a rather comprehensive breakdown of fox versus king coil overs, comparing the design and construction of just about every single component. You all can judge for yourselves if the differences are significant enough for you.
Radflo coil overs are unique in that, although many consider them to be a step-down quality wise, from my research they likely offer the most specifically tuned performance available for the 5th gen (thanks in part to the work from Overland Warehouse).
As far as the distinct lack of Fox on this forum.. couldn't tell you. Icon seems to have a firm grip on this community. It looks like they make great products, but so do total chaos, camburg, and radflo. That said, there is something to buying a full "system" like Icon, with the reassurance that everything has been engineered and tested to work together, and some people will pay for that. As an aside, the Jeep community I used to be involved with revolved entirely around Kings, literally no one ran anything else.. so it may just be one of those social/herd mentality phenomenon's. Fox's seem to be more popular on the FJ-Cruiser and Taco boards. I had intended on going King or Fox and eventually went with Fox simply because I was offered a killer deal on them.. works for me.
I just got them and have only played with the adjusters once so far. Factory set, both in center, rides great. I have yet to take them on a nice brutal washboard, we will soon see.
