As far as this part of the question goes, from what I've seen, few if any people who have left CO for AA have been unhappy with their choice-- I can't think of a single example here-- but there must be a few. However the vast, vast majority of people who have left CO for AA share the one, single regret that I have-- I'm sorry I waited so long to leave.

That having been said, it's a pretty informed bunch on FT, so, if someone leaves an airline, they probably already have a pretty good reason and therefore are unlikely to regret the choice. In my case it was over as soon as they implemented the international upgrade policies. Left soon thereafter, and never for a second regretted the decision. From what I see on the AA board, all of the former CO fliers are very satisfied with the move to AA-- most are ecstatic to be done with OnePass and it's duplicity.

You shouldn't leave just for the sake of leaving, obviously-- or because other people have, etc.