We use CI 15 in our 320/319s and descent speed is often 250 knots which is far too slow to fit in to the general ATC environment (mind you it would work if everybody used it) so most people tend to use 280 which apparently makes no difference at all to the overall CI cost/time/fuel calculation. To make that work though you need to re-program the descent before you start down to show 280 knots. Going down at the 250kt top of descent and then selecting 280 and getting low or using v/s to stay on the profile is very inefficient.
As topslide says the whole slow descent thing is based on the fact that you spend less time in the cruise using fuel because the descent starts a few miles earlier but the number of times you are asked to speed up or get a few extra miles added to your track because you are put behind other traffic means that overall I am quite sure that entering a slightly higher speed makes good sense. There is no doubt that we have saved fuel by using a lower index but mainly due to slower cruise and taxing in on one engine, slow descent is not saving much at all.