I have been a fan of story points ever since I attended a seminar several years
ago where Mike Cohn presented the concept. I never really trusted other
estimating practices such as function points and time-based estimates. Why?
Software development projects are rarely similar from project to project, yet
these practices focused entirely on past experience. Therefore, to get a
"reliable" estimate of time for every new project you needed to gain a lot of
experience. In other words, you have to figure out up-front how you will do the
work. Not only does this take a lot of investment up front, it also does not
account that the work you do later could change based on the work you do now.
The estimates assume that nothing will change in the effort of doing the work,
which is definitely not true in the Agile world. What I like about Story points
is the focus is on the relative size of "things", then how they will be
accomplished. As Mike would say, "Estimate size now, derive duration later".
Read more here: http://leanagile.blogspot.com/2007/05/case-for-story-point-estimates.html
Good stuff, be sure to check out the rest of Skip Angel's posts while you're at it!