Ken- I do this sometimes in my T5. Usually when i'm exiting a TP or other expressway and i am going a bit too fast. My manual says only try it upto 70mph. I agree with Steve F. that it may heat up the tranny more(so that's another reason to flush it every few years). When i downshift, my tach races to about 3-4K rpms and i'm pushed back into my seat. I hold it there for a few seconds(5-10), then I re-engage "D"- all the while applying the brakes. this has saved my hide a few times, but I try not to make it a habit. As for hills, i would not/do not use it to brake or slow down. Just put it in "3" at the start of the hill, and keep you foot at the ready to hit the brakes.

CIAO,BD