Coming from a chess background (no do-overs in rated chess games) I don't like the undo feature. I've never asked for one, and never allow it if asked for after cards are drawn or future cards viewed, I usually allow it otherwise unless the opponent asks for it excessively. It's definitely the cause of a lot of strife and I'm sure that players' blacklists are much fuller because of the undo feature.
In my opinion, Dominion is the best game invented since Chess, it's so subtle, there is some luck involved but skill plays a much bigger role. I think the undo feature does somewhat of a disservice to the game, there's kind of a pervasive do-over mentality.
In a rated game, my objective of course is to win if I can, not to help my opponent win.
I know that a lot of players don't like the undo feature. Perhaps a poll should be taken on whether players that play here want the undo feature or not.