In my never ending quest for the "perfect" holster I bought a SmartCarry and use it occasionally. I find it about as comfortable or uncomfortable as wearing an IWB holster, but in different ways. When using it for a tiny gun like my Ruger LCP it was less intrusive even than pocket carry. For my 642 it also works OK. My biggest concern, however, is that if you keep your pants tightly belted, so as to not have your pants sagging and having to pull up the pants frequently, then drawing your gun can be a bit of a challenge. Your pants have to be at least loose enough to slide your hand easily down into the holster to draw the gun, and for me, depending on what else is in my pockets, sometimes means I have to wear the pants too loosely to be really comfortable. But it does TOTALLY conceal a gun, even if you are wearing cotton shorts where pocket carry causes too big a bulge, and neither IWB nor OWB works easily.
I think it worthwhile to own a SmartCarry even if you only use it occasionally. If I ever decide to carry a second firearm, then I think I would be carrying it in the SmartCarry holster every day.