We are like Tim in that when we go to a new port, we'll do ship's tours. Not many ports left that we can get to and haven't been to. Once we are familiar with a place, then we tend to do our own thing. The only time we tried to prebook with a shore excursion company, they never got back to us. Most places, we are fine just walking around a tad.
But, if there is something we really want to to, then back to ship's tours. Years ago, we tried to do something on our own that we had done before, but the ship had booked the entire maximum number of people, so we couldn't go. Learned another new lesson!