If you come to a complete stop on an incline and AS engages, the car should not roll back when you take your foot off the brake. However, I believe there is a time limit. One time I took my foot off the brake to see if the hold would stick and it only lasted about 1-2 seconds - just long enough to get your foot on the gas.

I have noticed, on a very slight grade, that it doesn't seem to stick and tends to roll a little bit when I take my foot off the brake. My suspicision was that the hold was dependent on a steep enough grade, but I don't have enough samples to justify this theory.

If your car is not holding, even on a steep grade, I would ask the dealer to look at it the next time you are in for service.