Organizations and information systems have been impacting each other over time. Organizations have become more and more complex concerning operating in an increasingly global environment. The development and evolution of information systems has established a utilization of very extensive and integrated systems which cover essentially all major organizational functions. These changes in organizations and systems set high demands on individual employees as users of such systems. In order to ensure the success of an organization, the information systems operated by its users should be acceptable both functionally and socially. The aim of this study is to present a holistic model on usability of information systems. We have used Jacob Nielsen's definition of information systems' usability as a starting point. In his model, Nielsen focuses on usability issues such as price and compatibility as well as factors of practical acceptability, e.g. learnability and memorability, only recognizing hard functionality and social acceptability as important factors without addressing them in detail. Our aim has been to develop a model that appropriately addresses also these issues excluded in Nielsen's model. Furthermore, we have pursued to develop the model to a tool that can be used in practice. Rather than trying to understand how a system supports its users concerning its functions, we have chosen to focus on the events in which a system does not do so. That is done by combining Nielsen's model with the theory of information systems' exceptions. To understand the issue of social acceptability, there exist a large number of papers and theories addressing individual user's reactions to computing. We carried out an extensive review of the literature, combined the theories and findings, and formed a set of issues most related to how a user is likely to react to any system. Understanding these user-characteristics with a system is a crucial step when interpreting acceptability analysis results. For our model to be functionally acceptable, a tool has been developed that can be used to perform comparable analysis of different systems in different organizations. This tool is a simple questionnaire to be filled out by individual users. This model has been tested in two pulp factories within same business group using the same system but with different user-experiences. This small scale study revealed interesting factors concerning the culture and practices of individual locations that have led to different subjective, usability results. Finally, this paper briefly discusses the theories our enhanced model is based on, presents an overview of the model, and concludes with a summary of the results of the initial study.