Precedence Effect

I don't use it much. Hit your interleave button on the master to hear why. I tend to rely on EQ and M/S compression to widen tracks, techniques that don't have mono-compatibility issues. The Haas trick can work but you've got to be careful with the delay settings: too short and you get comb filtering in mono, too long and you don't hear the Haas effect, just an echo.
 
I'm probably in the minority in this concern, though. And of course, there are certain genres where you don't have a choice as to whether to use it, because it's part of what defines the genre.