Sorry to say that the current version of similarity (1.5.3 build 1001) got less stable:
For large numbers of songs (>50K) the duplicate scan freezes from time to time, no CPU and file activity any more. When restarted, program stops at the same progress indication (xx.x%) again. When starting environment is changed (different directories or cache cleared) program sometimes arrives at 100%, sometimes stops elsewhere.
After successful search usage of GUI resources often fails: No reaction to ctrl-S or right click to spectrum analysis, no reaction to Enter or Play and other right click menu items. After some playing around (resorting, resizing etc.) ctrl-S suddenly works but after a few successful actions fails again.
Additional strange behaviour: Cursor shape does not change from border icon (N-S-arrow) to normal in client area, toggles rapidly back and forth between normal and N-S-cursor on action fields. Menu bar icons disappear and reappear whereas balloon tips and click actions function normally.