I've bought kindles for several different people, and in a couple of cases in the family, I've shared my account with them, at least initially, as a way of easing them into e-books. In one of these cases, I've developed more awareness of what the person was reading, but didn't find much overlap (not surprising). But in another of these cases, I'm finding that I'm reading a ton of what she's reading and, at times, tapping my foot waiting for her to finish so we don't screw each other up with whispersync.
I don't know if this is a long term situation. But I love it, and it's the closest I've come to a few beautiful years in my early 30s when I was trading paper bags full of trashy novels with a few friends.