I have tried each of the different FC 1-3 forms of this years model and the previous. Settled for the FC1 because it had he least pockets and i noted no difference in padding or comfort for me or my child by paying more. (I have not tried other carriers because Kelty is the only brand i can continently try at the small store in town.)
It carries children and not much else.
I do have two main complaints; the major one is the minimal padding for the adults comfort. I am an average man, 5 11" 156lbs and after 4 miles with my 25 pound son, my clavicles and iliacs hate me. When I put on a pack, I have come to expect the level of comfort offered me by an Osprey Argon 70 (or what ever you see as the grail of pack); this pack falls far short.
The second complaint is more of a note. The cargo pocket configurations will move the weight away from the core different from a traditional pack (out or down). Kid carries are not meant to carry much mass in the pockets. My balance was off for the first few times I had a water bottle and lunch back there. In addition, hydration compatibility would have been nice too, but a little work rigged a bladder in the mix fine.
Altogether is carries the kids great and they love it; bottom line, not for hours of fun just short trips