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.