Can't be easily attached to a mask
And that keeps it from being useful.  Other, similar, products have little "hook" areas that can be attached to a thread loop sewn into a cloth mask.  This one uses nothing but your nose and friction to hold it in place.  It's not that comfortable when a mask is adjusted tightly enough to keep this from falling out or slipping around.  It's also a nuisance if you need/can remove your mask on a regular basis as this must be kept separately and re-adjusted/inserted every time you put your mask back on.
Rating: 2 stars