This paper describes an experiment on the usability of a new personal information management system on internet (PIMI). The goals are to evaluate its usability, and to assess user tailoring as an evaluation technique. Thirty users participated to the study: a first part consisted in a standard user test (SUT) and a second part was a usability test with tailoring by users (UTT). Overall, a total of 51 usability problems were diagnosed. Among those, 32 resulted from SUT, and 19 from UTT. Part of the latter (11) are additional to the ones identified during SUT, and to those diagnosed previously by usability inspection (UI with Ergonomic Criteria). The active involvement of users through customization scenarios appear to provide additional cues for usability assessment, and for design (new generic usability recommendations).