Not so much a fan of the plotline but I did like Sam Rockwell's performance. Also liked Jeffrey Combs performance as various Weyoun clones, but that was more for his personality than the fact of him being a clone. The clone thing was invented as an excuse to bring back Combs after they killed him off.
The clone schitck does give a good actor a chance to play two different roles, but an evil-mirror-universe-twin scenario would do the same.
My problem with clones is that they're not very sci fi considering that there are millions running around planet Earth right now, and have been for eons: identical twins.
Waitaminute, I though of one scenario I liked...when Crichton was cloned. That was good. I liked that they didn't designate one of the clones as the "real" one, which implied the real Crichton died