The day I can book a flight, lets say the following with metal used in brackets, booked as an AC ticket:

YYZ-ORD(AC)-MSP(UA)-ORD(UA)-YYZ(UA)

Arrive 20 minutes early from MSP to ORD on the return leg, with no checked luggage, and get on the FRICKEN AC METAL WHO I BOUGHT THE TICKET FROM that's leaving in 20 minutes instead of waiting 2 hours for the next UA flight, I'll book a codeshare again.

United Agent: "Sir, we can't book you on another airline - would that make sense to you?"

AAAAAARRRRGGGHHHHH

I have no reason to believe this will be any different on CO

</rant>