I prefer photos over videos personally.
But, I'm not opposed to my partners watching videos and I have watched a few with them.
Generally I get stuck staring dumbfoundedly when the sounds, words, music whatever don't correlate for crap with the actions. That leaves me at a loss for interest and certainly not moving into "turned on" mode.
But, there have been a few over the years that I could lose myself in the obvious enjoyment of the participants.
It's not classified as porn, but the movie Better Than Chocolate was an assigned film for a class I just took this summer. I LOVED the sex scene in it between the two female main characters. It was so damn obvious by the facial expressions that they were enjoying the activity. THAT turned me on beyond belief and led to a great personal experience with my husband.
I get turned on by sincere enjoyment, it doesn't generally matter what it is that they're doing-as long as everyone whose participating is enjoying it. I can enjoy watching a BDSM scene, or lovemaking, guy on girl, guy on guy, or girl on girl, or more than 2 groups as well. But, I'm not excited if they aren't all VISIBLY enjoying the activity.