Emerging technologies have brought several new ways to track, measure and evaluate own activity. Well-being, nutrition, physical training, mood, and sleep are a few of the various measures that can be self-tracked by different technological solutions. At the same time, people are becoming more interested in themselves and their own well-being, and constant tracking of own activities is getting more and more popular both on individual level as well as in general healthcare. This study examines critical experiences that occur during the implementation phase of the innovation-decision process and their influence to adopting or rejecting a self-tracking technology. The study is qualitative in nature and empirically based on thematic analysis of ten semi-structured interviews used together with the critical incident technique. The theoretical basis of the study comes from two well-established technology acceptance models: unified theory of acceptance and use of technology 2 (UTAUT2) and innovation-decision process. The results reveal the experiences and factors that are important for users in terms of deciding whether to adopt or reject a self-tracking technology during the initial phase of implementation. The results are also reflected to UTAUT2 and the innovation-decision process.