The aim of this study was to investigate the opinion of authors on the overall quality and effectiveness of reviewers' contributions to reviewed papers. We employed an on-line survey of thirteen journals which publish articles in the field of life, social or technological sciences. Responses received from 193 authors were analysed using a mixed-effects model in order to determine factors deemed the most important in the authors' evaluation of the reviewers. Qualitative content analysis of the responses to open questions was performed as well. The mixed-effects model revealed that the authors' assessment of the competence of referees strongly depended on the final editorial decision and that the speed of thereview process was influential as well. In Ordinary Least Squares (OLS) analysis on seven questions detailing authors' opinions, perception ofreview speed remained a significant predictor of the assessment. In addition, both the perceived competence and helpfulness of the reviewerssignificantly and positively affected the authors' evaluation. New models were used to re-check the value of these two factors and it was confirmed that the assessment of the competence of reviewers strongly depended on the final editorial decision.