oops - not concave, I meant convex!
Well, it's a matter of taste/opinion, really. Some don't mind flat glass at all. I don't like it. You get a better, wider view with a slightly convex lens. In an exaggerated case, you can get those super-convex mirrors that just stick on your normal mirror. To me, those are TOO much.
JCWhitney has a few mirrors that are specified as convex. Most of their collection is flat-glass, and specifically marketed as such. Ugh. Unfortunately, the convex mirrors they offer aren't stock-looking at all. Black plastic, rectangular case. However, I've used them on two or three different bikes and I really like the long-stem version they sell. But, it was on bikes were I wasn't concerned about trying to look stock.
But, it's just my own preference. The flat-glass mirrors work, and certainly are better than NO mirror. If you are / have been happy with flat, go for it.
Kirk