ask ten different people, get ten different answers. I have the iwill ka266r (v1.3) and have had no problems, it runs better than my abit kt7 did. On the flip side, you could get ten of the same mobos and get diff. results with each one. I couldnt say one is better than the other right now, none of the boards are mature yet, but the ka266r works for me.
I think the general consensus at this point is that the AMD 760 and KT266 based boards are comparable in performance and that the Magik1 based boards are the dogs..... I'd wait for some of the big names to come out with KT266 boards (read: Iwill, Abit and Asus) before making a decision.