I don't think it will go through, partly because AMR is using it to sell Sabre reservations services which AC doesn't really want.
I am against it because it doesn't give AC anything, and basically bails out CP. The amount of debt on the company, and horribly mismatched fleet could be a major problem as well. One of its biggest problems is also to do with alliances. Look at AC's route structure. Going into OneWorld throws away its strengths, such as good connections in Frankfurt, and also the geographic location of Denver is much better than Dallas.
As far as the money for shareholders, I'm surprised nobody else has noticed this in the media or elsewhere. Claiming that AC stock has been flat in the last 10 years isn't quite fair. Look at the values 18 months ago. They based their offer on the last 52 weeks, but for the last year there have been factors dragging down the stock value. Note that the past year has included a pilots strike, a near miss on a flight attendants strike, and negotiations with the mechanics. And I think there are enough investors speculating that there will be a counter-bid that AC unless Onex sweetens its offer to $11 or $12 from $8.25, investors just won't be interested.