Well, I had a chance to try the thing out and I have to say that I am happier doing:
:r ! diff -r --brief
and then making my own quick macro to diff any files mentioned in the diff results. I just like the ability to filter out any lines I already know aren't a factor in my diffing. The DirDiff plugin doesn't allow you to change the contents of the 'diff results'. Well, I was at first. Until I figured out all I had to do was type:
Then I could delete the 'results' screen willy nilly. Much better. It still complained if I tried to insert text or what not...so its kind of a toss up. If I needed to quickly copy the contents back and forth then this DirDiff tool would be just the thing.
The trouble is most of the time I'm actually looking for a needle in the haystack. Some little change in a text file is causing a world of havoc for me. I don't want to filter out 'types' of files, and copy whole swathes of differences between the trees. In this case, I think manually reading in the diff and recording your own little quick diff macro is the way to go. DirDiff is slightly too constrained in what it wants to let you do.
In summary: a good plugin if you need to copy back and forth changes, or you don't want to spend the extra 3 minutes it takes to record your own diffing macro. I've rated this script 'Helpful'.