Well 3 out of 4 of my ET's i did use either 1 or 2 restarts or replans but only because after I started it I found a different approach I wanted to use, not because I screwed up.
As far as being replayable, I would LOVE it if they became available all as one single DLC release sometime after disc release, either free or super cheap. But I dont want them before that cuz it takes away the whole point of the mode.
Thats my 2 cents. Dammit, just got spotted.