My inside perspective from using this system is that, at least the way we use it, it is not useful for forecasting. In each sprint approximately 40-50% or so of the tasks actually get finished. Most of the rest we carry over, and occasionally a few will be deprioritized and removed.

The points values are not used very systematically. Some items aren't even assigned a point value. For those that are, the values do generally tend to correspond to amounts of effort of 'a day', 'a week', or 'a month', but not with very much precision.

We certainly don't focus on whether or not our points predictions were accurate to the amount of time actually taken during retrospectives. I think that in the past 12 months, all but one or two of our retrospectives have ended up being cancelled, because they aren't seen as very important.

Probably we are not utilizing this system to its fullest capacity, but our group isn't dysfuncitonal or anything. The system seems to work pretty well as a management tool for us (but is not so useful for forecasting!).