If a mirror is nearby I've sometimes looked at myself in it, as it gives me a different perspective of my hard cock in relation to my body. We all have the same view: looking down at our cocks, so seeing it from the side or full body in a mirror can be fun while masturbating.
As for sex, I've loved every opportunity where a mirror was involved. While all of my bedrooms I've had have had a mirror in it somewhere, most of the time it wasn't convenient to the bed. I remember staying at a hotel that had a wall mirror adjacent to the bed, and immediately thought "This is going to be good!" So did the woman I was with and we made sure to glance over and watch ourselves fucking. It's like a free porno movie starring yourself and your partner!
At another stay at another hotel, we ended up in the bathroom and I slide into her from behind. This bathroom had the full width mirror behind the sink and she quickly realized she could watch me fucking her, which turned her on even more. A turned on woman always turns me on, so that fuck in front of the mirror was extra hot.
A guy gets to see himself fucking in rear entry positions, but a woman really doesn't have the same opportunity, so a well placed mirror can add to both partners enjoyment