These are very, very cheaply made - even considering the relatively low price. This is the kind of device that you want to have some degree of confidence in, and these leave me with considerable doubts. In hindsight, I wish I had just accepted spending a few dollars per window - after all, these sensors will be there for years and years, you will want them to be quality/last.
A few other notes - I removed one, and the double-sided tape was very difficult to remove cleanly (I'm still at it). Even VHB tape (which is very strong) would remove more cleanly. Also, for double sash windows, you want to install these where the sashes meet, so that it will alarm when EITHER sash is moved. The size of these makes that impossible, unless your windows have exceptionally thick frames (need 0.5 inch frame protruding beyond the glass pane). So as-is, you would need one sensor for each sash if you want to detect both.