Plierwrench is far from a gimmick! In fact I've been looking for an inexpensive source for the 5" wrench for keeping in a pocket from time to time while working out in the yard or around the house.
In my opinion, the 5" pliers wrench is a great tool, but Knipex has a strange pricing scale. The 5" and 6" are basically the same price, around $40 on Amazon. The 7" is $48, 10" is $50, 12" is $68 and 16" is $158.
I bought the 10" pair first. I use it quite a bit for all sorts of things, and thought a smaller version would be handy, but I thought paying almost as much as my 10" was silly for a smaller tool. I got lucky and got a screaming deal on a 5" pliers wrench and 8" end nippers as a package, and quickly got over the price hang-up.
I use the 5" more than my 10" now. I have retired my 4" adjustable wrenches to the collection department. One thing i didn't notice until i got them was that the jaws are very thin...like 1/8"...and that makes them even more handy. It's like having a set of those crappy Oxwall stamped steel wrenches, only made of much better steel, and with plier handles.
I think that at $158, the 16" version is more of a gimmick. I think in that size a large adjustable would be a more practical, less expensive solution.