[HPsauce]

A couple of things I've noticed after rather more use:
1. The interior lights coming on (e.g. dropping off a passenger) make it go very dark very quickly. If you're using the interior mirror to check behind before moving off you need to wait briefly.
2. In the (admittedly very rare) situation of someone overtaking you on the motorway at night the darkest is actually as they get close and move out of your view. I think this is due to the direction the rear-facing sensor points in, i.e where the mirror is pointing which is obviously half-way between rearwards and towards your eyes, so significantly to the offside.

Oddly I haven't noticed either of these effects on the similar mirror in my Ford Focus, though the rear-facing sensor on that is recessed below the mirror rather than behind the top of the glass. And being a hatchback there's probably more bodywork blocking light from overtaking vehicles. (it also has a handy "off" button)