I have a couple FJs, and a "couple" of RMDs... and variants. I like them both. Also an Axe Hammer. The Flak Jack and Axe Hammer have practically the same grip and scales. In fact, the sheath I have for the Axe Hammer fit the FJs. In any case, FJs balance well, are quite sharp, come from Busse with thinner edges than the RMD, and just feel bada$$. But, for some reason, I seem to prefer the RMD as a general working knife. I always felt the choil for the Axe Jacks was oversized. I understand why (I think). I think it is to accommodate individuals that are almost always wearing some kind of gloves (high-speed/low-drag guys?). But for me, it just swallows my index finger. And for the most part, I prefer fixed-blades with finger choils on them.
The RMD is, for me, that knife that can do just about anything. Some things better, some things not as good, but its size and heft will let it work its way through.
The FJ is just a little too long, balanced differently, and just SEEMS like it would be more of a fighter than a general-use blade, or survival knife.