Fair question fishface. With my smallish, but sometimes fairly numb hands, the bone and Santa Fe Kiwis open one handed every time with no problems. The all stainless one, I beleive due to the fact that it is considerably thinner, is more problematic for me. When my hands are at their best, it works fine, at their worst, I'm reduced to opening that one with two hands, but that is still easier to accomplish via the hole than with any stud or nail nick "gents knife" I've ever tried. The bone one is my most often carried EDC for work days. For perspective though, I find my FRN and CF Dragonflys to be easy one hand openers too and, as with the Kiwis, the all stainless one to be more problematic.
One last thought, off the topic of Kiwis a bit. I found the almite Navigator slippery and thus difficult to open one handed despite the larger size hole it has. On the other hand, they're selling pretty cheap right now, and I'm acutally thinking of getting another one, removing the clip, roughing up the almite a bit with sandpaper, and epoxying 1/8" wood slabs on both sides, as I really do like that larger hole. Figure wood scales will add both a bit of thickness and a fair amount of increased traction without adding too much weigh.