I've done the exact itinerary and loved it (could have done without Istanbul but that is a personal preference). The ports are extremely interesting but be prepared to be tired. We went all out to make sure we made the most of it and often found ourselves falling asleep at 9 PM.
You might also want to take a look at some cruises that are more focused on the Greek Isles. In comparing the two we preferred that but wouldn't hesitate to do either.
As the other poster mentioned, cruise critic is a good site.
As far as Princess, they are a good line and the Ruby is one of their newer ships. Unless your tastes are 5 star, you shouldn't be disappointed.
We looked at the Princess itinerary, but decided to go with the Celebrity Equinox cruise from Rome, which is focused more on the eastern Med. Holland America also has some cruises that may interest you.
All 3 are good lines from our experience, so we focus on itinerary and price.
We were on the Crown Princess from Venice to Barcelona. We arrived in Venice 2 days early so we could tour there. Stops were Athens, Kusadesi, Istanbul, Mykonos, Naples, Rome, Florence, Marseilles (which we missed due to excessively high winds) and Barcelona. We stayed on an extra day in Barcelona. We have since cruised with Princess 3 more times and are booked for the British Isles in August.
As mentioned, prepare to be tired - long days and so much to see. Some advice:
As much as possible book your tours privately, not through Princess. You will see more, have more flexibility and probably more fun. You could even save a few sheckles. If you can, book in advance, and if you can get others to go with you, save enen more. At worst, don't take the organized tours - book cars/van through Princess. Remember you're only as fast as the slowest member of your bus group. We missed some of Ephesus because an older couple were unaare that Ephesus is difficult to walk. We slowed down to insure they were included, but missed parts of the tour. SImilarly, we took the bus tour of the Amalfi coast and sat on the wrong side of the bus (sit on the right side). Had we taken a private car we could have stopped more often and seen so much more. A private operator will get you into the Vatican faster than the tour group. We learned the hard way and now do everything ourselves.
We do love Princess though. We've always taken a balcony and HIGHLY recommend spending a few $ more to get a cabin overlooking the very rear of the ship - the views as you leave each port can be tremendous. Plus, the balconies are a bit bigger - much better for 4 to enjoy happy hour.
Book late dinner - you'll want to rest after touring all day.
Princess allows you to bring wine on board (used to be each day - not sure if still the case). We got some very nice and inexpensive bottles of wine in Venice, Mykonos and Amalfi and were able to enjoy wine at happy hour on the balcony. Just add cheese from the buffet!
If you don't care for formal night, book dinner at the Crown Grill or Sabatinis and take a sports coat. I've done all the formals I can handle. If you do decide to go formal, book your tux through Princess - it will be delivered to your room as you arrive and picked up after the final formal night. MUCH better than taking your own, and inexpensive. Get a local men's store to measure you before you book.
I've blathered on enough - I hope this helps. have fun - it's wonderful trip.