One of the pieces of feedback we frequently get is that creating custom reports is too hard.  We're taking a two pronged approach to this.  We're working on new features in Rosario to make it easier and we are writing more/better docs on how to do reporting with the features you have today.  Here is a post from Allen Clark on some new docs for writing TFS reports using Report Designer.  Check it out...

http://blogs.msdn.com/vstsue/archive/2007/08/29/new-content-available-on-creating-custom-reports.aspx

Brian