Fairly rare from the sound of it. Most of the stories I hear about the rare encounters with other players tend to be along the lines of "A guy in full power armor ran by, and stopped for a moment to point me to where he'd dropped a pile of items that were worthless for him but 20 levels higher than anything I had." And if you do have annoying players around, all you need to do is log out and back in and you'll almost certainly end up in a different instance.