Despite the increasing implementation of formative assessment in medical education, its' effect on learning behaviour remains questionable. This effect may depend on how students value formative, and summative assessments differently. Informed by Expectancy Value Theory, we compared test preparation, feedback use, and test-taking motivation of medical students who either took a purely formative progress test ("formative" PT-group) or a progress test that yielded study credits ("summative" PT-group). In a mixed-methods study design, we triangulated quantitative questionnaire data (n = 264), logging data of an online PT feedback system (n = 618), and qualitative interview data (n = 21) to compare feedback use, and test-taking motivation between the "formative" PT-group (n = 316), and the "summative" PT-group (n = 302). Self-reported, and actual feedback consultation was higher in the "summative" PT-group. Test preparation, and active feedback use were relatively low and similar in both groups. Both quantitative, and qualitative results showed that the motivation to prepare and consult feedback relates to how students value the assessment. In the interview data, a link could be made with goal orientation theory, as performance-oriented students perceived the "formative" PT as not important due to the lack of study credits. This led to low test-taking effort, and feedback consultation after the "formative" PT. In contrast, learning-oriented students valued the "formative" PT, and used it for self-study or self-assessment to gain feedback. Our results indicate that most students are less motivated to put effort in the test, and use feedback when there are no direct consequences. A supportive assessment environment that emphasizes recognition of the value of formative testing is required to motivate students to use feedback for learning.