For the most part the 2nd Generation contact sensors are OK.  However, on several of my windows the available space on the windows and the window frames is too small to accommodate the sensor.  That is neither the magnet nor the sensor will fit in the available space without creating some issue.  On some of my windows the window itself has narrow wood separators between the glass panes and also at the edge of the window where it closes (these are crank-out windows, not slide-up windows).  So, it is possible to just barely attach the magnet on this part of the window and have it open and close without issue.  The matching sensor then needs to attach to the window frame which, on my windows, are shallow and do not have much space for adherence.  What is even worse, my window screens attach inside this area and when the sensor is placed as described the sensor body projects too far back toward the inside of the house to insert the window screen in its mounting holes.  It would seem that the physical placement of the 2 batteries for the sensor is what creates the large size of the sensor body.  Maybe a future design could stack the batteries atop the circuit board and thus make the sensor body less deep, albeit somewhat thicker.