I have the 3.5" screen and have mounted it what is the most natural spot for a 'mirror replacement'
In that spot, glare from the sun isn't a problem (windows are tinted though) and it's only slightly visible on the windshield at night, and even then only if I"m sitting all the way back. I haven't driven a lot of miles (<100) like this though, so I'll see how it does in different weather conditions.
I'm impressed with the field of view on this thing though! I can see the side of my car and someone directly beside of me at the same time.