There seems to be some kind of a gap between users and developers as far the eagerness to abandon PDB in favour of mmCIF. I myself fully agree with Jeffrey about the ease of manipulating PDB's during work, particularly when encountering unusual circumstances (and there are many of those, as we all know). And how about non-crystallographers that are using PDB's for visualization and understanding how their proteins work? I teach many such students and it's fairly easy to explain to them where to look in the PDB for particular pieces of information relevant to the structure. I can't imagine how they'll cope with the cryptic mmCIF format.