This is one of the best articles I’ve ever read on software estimation and engineering project management. My favorite paragraph:
The intent of story points is to focus on relative sizing independently of other expectations of time. The problem is that, in being numeric, story points become a currency susceptible to arithmetic, especially once they’ve been aggregated. Rather than the approximation an estimate is intended to be, story points invite conversion while offering the illusion of precision. Once a conversion rate is established, the converted value becomes how people think and talk about them, whether tacitly or explicitly. Story points degenerate into a measure of time, albeit one abstracted enough from real time that, like any currency, they become subject to speculation and inflation. Although a looser fit, T-shirt sizes (S, M, L, XL, etc.) can also fall into the same trap.