Story Points Aren't Accurate - That's Why They're Good
Ben Northrop wrote to complain that story points are not accurate. They don't (always) map linearly to hours spent, so adding up story points over a large project won't accurately give hours for the project. In the spirit of expressing controversial opinions, I will agree, and explain why I think that's a good thing.
I believe that story points serve as a "rough" estimate. In the teams I work with, story point estimates are made quickly (a few minutes to be sure we understand the story, then quickly discuss and reach a consensus estimate). They are quantized (must round off to some Fibonacci number) which means that any given estimate is necessarily imperfect.
As such, they provide a cheap (didn't take long to generate) but rough (not perfectly accurate) estimate, and they have to be respected as such. Story point estimates would not be useful to answer questions like "Will this project deliver in October or November?", but they ARE useful for questions like "Would this be a 3-month project or a 1 year project?" For some purposes, a more precise estimate is needed, and then it may be necessary to invest a few hours to a few weeks to perform detailed work to generate a more precise estimate. However, I think that such situations are rare: people *want* perfect estimates ahead of time but rarely *need* them. Also I think that people are usually fooling themselves: most (usually waterfall) projects with precise up-front estimates later discover that those estimates are not accurate.
One of the strengths of story points is that everyone (including the customer) REALIZES that they are rough and don't correspond to a precise delivery date -- something that can be difficult to explain for estimates expressed in hours.