It's worse than useless. Not only is the view is too small to notice cars in the blind spot naturally without looking much more closely, it covers up a portion of the mirror that I would normally use to see my blindspot. The end result is that I have the driver side mirror in the standard orientation and I use the crappy convex mirror to check my blindspot, but since I can't see them well enough I have to turn my head and physically check my blindspot. Luckily they didn't ruin the right side mirror so I can still see everything without having to look any farther back than my passenger window.

Trust me, this way is much better than convex mirrors. Try it.