I got back a couple of weeks ago from my 18 day (13 cities) trip that included an 11 day cruise.
Overall, it was an amazing trip and I would highly recommend it to anyone. It was my first trip across the Atlantic and we thought it would be a great way to see so many places. Here's my quick review of the stops.

Paris - 6.5 layover so we raced into town to see the sites quickly. I really liked Paris and plan on going back. The architecture is really cool.

Florence - After a night's stay in Rome, we took a train to Florence. Beautiful city and a great city to walk. Enjoyed just eating and drinking outside and taking in the scenery. We toured the Boboli Gardens which were awesome and went to the Da Vinci museum.

Venice - Took a train from Florence to Venice. I heard mixed reviews about this place but I thought it was neat. It's unlike any place I've seen and you will do a lot of walking to see everything. It does look a little run down in spots but I really appreciated the uniqueness of the place

Dubrovnik - This place had some incredible views. We did the GoT tour and it was a blast especially since our guide had some inside scoop since he was an extra on the show

Kotor - Probably the most scenic place as you arrive in the port. We hiked the Wall of Kotor which provided some amazing views of the area. The city itself is rather poor but very unique. It's cool to learn the history of the place and how it had so many different influences

Corfu - This was the hidden gem of the trip. It wasn't commercialized and had a ton of natural beauty. We did a bus tour that took us around the island.

Santorini - This was a cool place with the city essentially built on the cliff. They offer donkey rides up the cliff. The path really smells and the donkeys don't really cooperate.

Mykonos - Probably the most disappointing place. I heard so much about their world famous beaches. We did the beach excursion and was very disappointed. The port area was scenic but there wasn't much too it. I will not be going back there.

Athens - I wasn't a fan of the city. I was glad to see some of the historical spots but it was very crowded and dirty. I will not be going back there

Sicily - This was my favorite spot as I didn't know what to expect. We were on the East Coast (Taormina) and this place was visually stunning. This is what I expected Naples to be like. Seeing the active volcano (Mt. Etna) in the background with some amazing scenery was something to behold.

Naples - I had high expectations for this place but I was very disappointed. We did a bike tour around the city which was a mistake. We heard the Pompeii tour was amazing as well as the excursions to the Isle of Capri and Sorrento. The city was a bit dumpy and I was expecting a luxurious beachfront town.

Pisa - This was great for a day trip. We climbed the Tower of Pisa which was very cool. We had lunch in the town but there wasn't much to see but I was glad we went for just the day.

Rome - We finished the trip with 3 nights in Rome. I really enjoyed Rome as there was so much history and beauty it seemed around every corner. The only downside of this city was the volume of peddlers.

General Thoughts of Europe:
-Too congested on the roads with so many mopeds. It's like NYC but with tons of mopeds
-Most places charged for using the bathroom unless you ate at the restaurant
-The French people lived up to their reputation
-Italy was awesome. I couldn't find a bad meal there.
-Norwegian Cruise Line was fine. Not the greatest food but the staff was great and it served its purpose to get us to the various ports
-The water of the Mediterranean and Agean Sea is a spectacular blue. A huge change from what we see off the East Coast here
- September is the best time to go. Not as crowded or as hot as the summer and the weather was perfect. It rained once on our 18 day trip.