HDC works pretty well. I've used it mainly in dry and semi-muddy conditions.
You can start using it anytime, part ways down a hill, before the hill, etc.
If you leave it on though it doesn't last all that long, I am guessing but maybe it lasted just over 0.75 mile the last time I ran it and just left it on. Granted a pretty good grade for awhile and I was testing it by giving it gas and then braking to see how well the 'HDC' maintained speed. Before the HDC turned off I got bells and a warning on the dash saying Driver must take control. With that said my foot was ready to take over if and when needed.
Haven't tested it on snow/ice yet, but so far I like it pretty well. IMO - its a great feature and I would add it to my other vehicle's I take off road as well if I could, but again that opinion may change once I test it on snow and ice.