I had incredible difficulty with the rewind/undo interface. Almost as often as I bring up the rewind UI, it'll do some other action instead, like drawing little dots, or zooming in/out, or bringing up/dismissing the tools palette.

When the dialog is finally brought up, it doesn't respect my finger motions. I'll drag my one finger a full 360 around my other finger but the pie chart moves less than 5 degrees in total, jerkily. Or, it flaps forward and backward rapidly as I smoothly move my finger in one direction. Maybe there's a heuristic that's trying to pick a "pivot" finger and a "rotating" finger, and it's picking wrong each time. I dunno.

Willing to give them the benefit of the doubt because the rest of the app is so well done, but, this brokenness of such a key feature makes a bad initial impression.