This paper presents a study on the usability of a new prototype system for managing personal information on the Internet (PIMI). The objectives are to assess its ease of use, and assess user modification as an evaluation technique. Thirty users participated in the experiment: the first part was a classical usability test (TUC) and a second part was a usability test with user modifications (TUM). A total of 51 usability problems were diagnosed. Among them, 32 with TUC, and 19 with TUM. Part of the latter (11) adds to those identified with TUC, and those previously diagnosed during a usability inspection (IU with Ergonomic Criteria). The active participation of users through customization scenarios seems to provide additional clues for evaluating usability and for design (new generic usability recommendations).