Originally Posted by JohnnyP
In my experience, they book into Y, both on UA and on other airlines.
I have the same experience. It's usually Y. Although I've had a couple times when I've been booked into a bona fide C or even F class.
On UA the bigger bonus usually kicks in. On CO, I've gotten double segment value (two, instead of just one "point" per segment for status purposes). Perhaps this makes it "triple-dipping" instead of merely "double-dipping." Yet another benefit of getting bumped.