This paper presents an analysis of user studies from a review of papers describing new visualisation applications and uses these to highlight various issues related to the evaluation of visualisations. We first consider some of the reasons why the process of evaluating visualisations is so difficult. We then dissect the problem by discussing the importance of recognising the nature of experimental design, datasets and participants as well as the statistical analysis of results. We propose explorative evaluation as a method of discovering new things about visualisation techniques, which may give us a better understanding of the mechanisms of visualisations. Finally we give some practical guidance on how to do evaluation correctly