It's thin in viscosity but not inferior in any way other than that in terms of shrinkage, compressive strength, or other relative measures.
Devcon is popular because it provides all the "right" properties, out of the tub without having to modify it.
I add fillers when I use JB (though I usually use West System now)- but I would NEVER use microballoons.
They have zero structural strength, and are used only as a fairing filler for easier sanding. Colloidal silica, or high density fillers, or powdered aluminum or steel are best suited for this application IMO.
http://www.westsystem.com/ss/filler-selection-guide/