Addendum December 2012: Have now 3 of these mice, 2 still in use. The first of 2010, but now broken, the left button makes only double clicks (no, that is not set by mistake in Windows so). Nevertheless, I would buy this model again and again. Even the MacBook Air I sacrifice rather one of the only 2 USB ports for the Unifying receiver instead of using the specially-selling Bluetooth Mouse Logitech V470 - which is simply inaccurate under OS-X. By contrast, the M505 fail on PC and Mac.