I find hill assist generally kicks in on a steep incline which is where it is most useful. I thought I would find it annoying but don't mind it all to be honest, particularly when you are in real tight with someone hanging on your tail.
2013 Boss 302
2012 Mustang GT Grabber Blue (sold)
401A, MT82, Brembo, 3.55, HID & Security, Electronics Pkg