At my current project, we've started to initiate a workload estimation based on Use Case Metrics. Apparently several methods exists, but we chose the Gustav Karner methodology. I'm quite impressed with how environmental and technical risks are translated into a factor that can be applied on an initial manhour estimation. Shivprasad koirala wrote a great article that clearly explains the fundamentals of this technique, and Roy Clem provided a summary. As soon as we have real-life metrics I'll update this post. Notice that Sparx System's Enterprise Architect provides full support for Use Case Metrics (including the above mentioned factors).