I have not… yet.
I really like the BFK. Its attributes that make it fall short of the FK2, for me, are basically all the changes that the FK3 option addresses.
Of course, part of it is just that I’ve taken the FK2 on most of the extended hikes. So it’s familiarity more than anything. My HDFK used to hold this position. Haven’t had a chance to try my SDFK on any multi days yet.
Truth is I’ve never “needed” my knives on any hikes. More of a convenience. Great for fire building but haven’t actually been in a situation where I required building a fire for survival. I’ve been in situations where there have been big temperature changes at sundown as I’m finishing up the days hike. I didn’t bother with a fire, just got the tent set up real quick, and jumped inside to warm up and put on extra layers. Then got out and cooked on the camp stove and then back into the tent.
Not saying I could get by without a knife at all. Just that my leatherman or SAK would get me through skinning my meals (by which I mean opening the camp meal bags so I can pour boiled water in).