While we haven't been on the Star Princess, we have been on the Crown Princess and several others in the Princess fleet. I've heard some people complaining about the Crown, but our personal experience was very good. We loved the ship, the food, and especially the staff. Of course, this was a few years ago and as we all know, things always change in this industry, so it's possible it's different from when we went.
Also, as we all know, food is very subjective. My favorite expression is; what one person loves, another will hate. And food is proof that statement is correct - everyone has their own opinion.
We thought the food was good. Just our personal opinion, obviously, but we felt it was definitely better than Carnival, NCL, or RCCL, but not quite up to Celebrity or Holland.
As for the shows, they were fine - nothing special - just the usual cruise line show.
As for Zumba classes, can't help you with that specific class, but I do know they offer classes - just not sure which ones. As popular as Zumba classes are, I would venture to guess they probably do have them.
As for the ports, not much to do in Grand Turk - basically a day at the beach right beside the pier. It was built and designed for Carnival Corp, so they want to keep you right there, but there are a few shore excursions.
Princess Cays is very nice! It's a beautiful beach with all the amenities. They have sandy beach areas and down past the gazebo, there is some decent snorkeling (not great, but nice).
St. Thomas is an American port known for its shopping. You can get some good deals especially on jewelry, watches, cameras, etc. But don't shop right at the pier - prices are often higher and they don't negotiate the way they will downtown. And since I'm on the subject, don't follow the crowd and be herded on taxis with everyone else. Walk outside the gated area and flag down a taxi - cheaper and MUCH less crowded. (Taxis inside the gated area have to pay a fee to be there and pass this along to passengers.) St. Thomas is also known for water sports of all kinds; scuba diving, sailing, jet skiing, etc. Magen's Bay was selected by the Travel Channel as one of the top 3 prettiest beaches. And while it's great for a sandy beach, it's not good for snorkeling. If you prefer snorkeling to just swimming, then Coki Beach is better. Or even better yet, you can take a water taxi over to St. John and try Trunk Bay.
But personally, I really like this ship and itinerary as it has the opportunity to enjoy a nice cruise while getting to sit and relax on some very pretty beaches. Two things we really love to do!