I get irritated with the constant quality setting, especially for high compression. From what I understand, the quality is constant within the video source, not between different sources.
I realize video compression is one of the black arts, but I would love to have a "set once" compression option to batch convert all my movies for a single target (phone for example). With either bit rate or CQ, there are always some videos that could be compressed more or really need to be compressed less to get them to the same quality to my eyes.
In answer to the question above, for DVDs I don't even bother to re-encode them any more since HD space is so cheap. Blu-rays I tend to keep on disc, at least for now. Going along with my theme above, I'm not sure there is a bit rate you can point to, at which point you have hit diminishing returns. It really depends on the source, the viewing environment, and the viewer.