Peer assessment has long been used in education as a tool to improve collaboration, critical thinking, motivation, and also students’ academic performance. In turn, self-assessment promotes the development of skills for self-regulated learning and allows for personalized instruction. In this study, we focus on in-service teachers who regularly assess their students in their daily practice. In the context of the continuing professional development in informatics in which they are enrolled, we examine their ability to evaluate the work of colleagues in an area new to them and also to reflect on their own work. Using qualitative analysis, we identified two key components of their verbal formative evaluation - the cognitive and affective components - and used them to assess the quality of their comments. The findings also show that the time taken to produce an assessment does not affect the quality of assessment comments as much as the way in which the comments are written. Comments written electronically were of higher quality than those written on paper.