I'm thinking this is going to be more like the Athlon or the Athlon 64 than the Phenom or FX. Just having Jim Keller around for this new microarchitecture should mean that AMD is able to get it right the first time around. They had a strong design team with the Athlon and A64 and nailed them right out of the gate.
The big question is going to be whether or not they can stay with the script after they launch it. The later iterations of the Athlon were outstanding, but they lost the script with the A64 and eventually lost the performance lead when Conroe came out. If Zen comes out and it's equal to Haswell or Broadwell, like people who have seen it are saying, that puts AMD back in the game. But if they can't develop it significantly in future releases, it won't matter. By the EOL of the K8, you could overclock the crap out of one and it wouldn't touch a C2D. Right now, you have to OC the crap out of an 8x0K just to get into the ballpark with a locked-down Sandy i5 from five years ago.