I recently bought and installed one of these convex mirrors, 11"x3" K-source (from Autozone). It is well-made, attaches nice and snug, and doesn't interfere with the visor. Although from the mirror I can see the rear side windows and a bit of the side rear door windows, I'm not sure it provides additional sight if you already have side view mirrors adjusted properly. And it is quite dim and of course everything is scaled smaller, so I'm not sure if I will keep it installed, will give it a go for at least a couple weeks.
Update... after only a couple days, I removed the convex mirror. It provided no benefits and a few negatives (dimmer view, smaller size objects, confusing), my side view mirrors easily beat it as far as blind spots and much clearer.