It depends heavily on your status -- as well as fare type and several other factors.

The top-tier elite (75K or higher) flyers on Northwest have been reporting a 92% upgrade rate, which is similar to most of the other majors. One notable exception is CO, whose Platinum members have reported a 69% upgrade success rate.

Log in and share your recent upgrade success (or lack of) at http://www.upgradesuccess.com/