We booked a private car and tour guide through
DCL when we were in Kirkwall, Scotland. I only did it, however, because we booked the cruise very last minute and all of the private guides in Kirkwall were completely booked and none of the other DCL group excursions covered everything we wanted to see there. If I could have booked privately, I would have as DCL's private car and guide pricing is, typically, substantially higher than what you would pay if you booked on your own. Our guide and driver met us outside the ship at the port. They would have taken us wherever we wanted to go, but, we let the tour guide make suggestions throughout the tour. I think YMMV depending on who is assigned as your driver and/or guide. In our case, both our driver and guide were absolutely fantastic and we had an incredible day with them. That being said, typically, if we want to do a private tour at a port, I do research on places like tripadvisor to get reviews on private guides and book them on our own.