I have a somewhat different perspective than Theresa. I opted for the "Grand Tour of Dominica" and enjoyed a fantastic tour of the island, including the emerald pool in the rain forest, with a very knowledgeable guide. This tour did consume a full day (it was the first tour to leave the ship and the last to return to the pier), but it definitely was very interesting and well worth the price. Of course, it might not be everybody's cup of tea.
I have taken six cruises with Princess, and have found the shore excursions to be organized very well and conducted by knowledgeable guides. The descriptions in Princess's shore excursion brochures are very reliable, and Princess takes great pains to ensure that they stay accurate -- even dispatching cruise staff randomly on shore excursions to verify that the excursions do match the descriptions and do conform to the company's high standards.
My recommendation is to read Princess's descriptions of the shore excursions carefully and to choose whichever excursion meets your interest.