Premium excursions are YMMV - we've had the worst excursion ever on Corfu that was a paid excursion. Privately arranged one can be good if you know of a good guide otherwise it's YMMV too - we've not done that. The advantage of using a cruise line excursion is that they take responsibility for getting you back to the ship before it sails - no such responsibilities if you arrange the excursion privately.